Highly oriented graphene structures and process for producing same

ABSTRACT

A process for producing a bulk highly oriented graphene structure, comprising: (a) preparing a graphene oxide dispersion having graphene oxide (GO) sheets dispersed in a fluid medium; (b) dispensing and depositing the dispersion onto a surface of a supporting substrate to form a layer of GO, wherein the dispensing and depositing procedure includes subjecting the dispersion to an orientation-inducing stress; (c) removing the fluid medium to form a dried layer of GO having an inter-plane spacing d 002  of 0.4 nm to 1.2 nm; (d) slicing the dried layer of GO into multiple pieces of dried GO and stacking at least two pieces of dried GO to form a mass of multiple pieces of GO; and (f) heat treating the mass under an optional first compressive stress to produce the highly oriented graphene structure at a first heat treatment temperature higher than 100° C. to an extent that an inter-plane spacing d 002  is decreased to a value less than 0.4 nm.

FIELD OF THE INVENTION

The present invention relates generally to the field of graphiticmaterials and, more particularly, to a new form of graphitic materialherein referred to as the bulk highly oriented graphene structure (notthin film) and the process for producing such a structure. This newgraphene-derived material exhibits an unprecedented combination ofexceptionally high thermal conductivity, high electrical conductivity,high mechanical strength, and high elastic modulus.

BACKGROUND OF THE INVENTION

Carbon is known to have five unique crystalline structures, includingdiamond, fullerene (0-D nano graphitic material), carbon nano-tube orcarbon nano-fiber (1-D nano graphitic material), graphene (2-D nanographitic material), and graphite (3-D graphitic material). The carbonnano-tube (CNT) refers to a tubular structure grown with a single wallor multi-wall. Carbon nano-tubes (CNTs) and carbon nano-fibers (CNFs)have a diameter on the order of a few nanometers to a few hundrednanometers. Their longitudinal, hollow structures impart uniquemechanical, electrical and chemical properties to the material. The CNTor CNF is a one-dimensional nano carbon or 1-D nano graphite material.

Bulk natural graphite is a 3-D graphitic material with each graphiteparticle being composed of multiple grains (a grain being a graphitesingle crystal or crystallite) with grain boundaries (amorphous ordefect zones) demarcating neighboring graphite single crystals. Eachgrain is composed of multiple graphene planes that are oriented parallelto one another. A graphene plane in a graphite crystallite is composedof carbon atoms occupying a two-dimensional, hexagonal lattice. In agiven grain or single crystal, the graphene planes are stacked andbonded via van der Waal forces in the crystallographic c-direction(perpendicular to the graphene plane or basal plane). Although all thegraphene planes in one grain are parallel to one another, typically thegraphene planes in one grain and the graphene planes in an adjacentgrain are inclined at different orientations. In other words, theorientations of the various grains in a graphite particle typicallydiffer from one grain to another.

A graphite single crystal (crystallite) per se is anisotropic with aproperty measured along a direction in the basal plane (crystallographica- or b-axis direction) being dramatically different than if measuredalong the crystallographic c-axis direction (thickness direction). Forinstance, the thermal conductivity of a graphite single crystal can beup to approximately 1,920 W/mK (theoretical) or 1,800 W/mK(experimental) in the basal plane (crystallographic a- and b-axisdirections), but that along the crystallographic c-axis direction isless than 10 W/mK (typically less than 5 W/mK). Further, the multiplegrains or crystallites in a graphite particle are typically all orientedalong different directions. Consequently, a natural graphite particlecomposed of multiple grains of different orientations exhibits anaverage property between these two extremes (i.e. between 5 W/mK and1,800 W/mK).

It would be highly desirable in many applications to produce a bulkgraphitic structure (containing single or multiple grains) havingsufficiently large dimensions (length, width, and thickness) and havingall graphene planes being essentially parallel to one another along onedesired direction. In other words, it is highly desirable to have onelarge-size graphitic structure (e.g. a fully integrated layer ofmultiple graphene planes) having the c-axis directions of all thegraphene planes being substantially parallel to one another and having asufficiently large length, width, and thickness for a particularapplication. Up to this point of time, it has not been possible toproduce such a highly oriented graphitic structure. Even though someattempts have been made to produce the so-called highly orientedpyrolytic graphite (HOPG) through tedious, energy intensive, andexpensive chemical vapor deposition (CVD) followed by ultra-hightemperature graphitization, the graphitic structure of the HOPG remainsinadequately aligned and, hence, exhibits properties that aresignificantly lower than theoretically predicted.

The present invention is directed at a new materials science approach todesigning and building a new class of materials herein referred to asthe highly oriented graphene structure (HOGS). A HOGS is a bulkstructure composed of highly aligned graphene or graphene oxide planes,wherein all of the graphene or graphene oxide planes are essentiallyparallel to one another. These graphene planes are much better alignedthan what the conventional HOPG has been able to achieve. Such a HOGShas a thickness typically at least 100 μm, but more typically at least0.5 mm, and most typically at least 5 mm. In most cases, the HOGS has anoxygen amount of 0.001-5% by weight, but can be essentially oxygen-free.The conventional HOPG contains no oxygen.

The constituent graphene planes of a graphite crystallite can beexfoliated and extracted or isolated to obtain individual graphenesheets of carbon atoms provided the inter-planar van der Waals forcescan be overcome. An isolated, individual graphene sheet of carbon atomsis commonly referred to as single-layer graphene. A stack of multiplegraphene planes bonded through van der Waals forces in the thicknessdirection with an inter-graphene plane spacing of approximately 0.3354nm is commonly referred to as a multi-layer graphene. A multi-layergraphene platelet has up to 300 layers of graphene planes (<100 nm inthickness), but more typically up to 30 graphene planes (<10 nm inthickness), even more typically up to 20 graphene planes (<7 nm inthickness), and most typically up to 10 graphene planes (commonlyreferred to as few-layer graphene in scientific community). Single-layergraphene and multi-layer graphene or graphene oxide sheets arecollectively called “nano graphene platelets” (NGPs). Graphenesheets/platelets (NGPs) are a new class of carbon nano material (a 2-Dnano carbon) that is distinct from the 0-D fullerene, the 1-D CNT, andthe 3-D graphite.

Our research group pioneered the development of graphene materials andrelated production processes as early as 2002: (1) B. Z. Jang and W. C.Huang, “Nano-scaled Graphene Plates,” U.S. Pat. No. 7,071,258 (Jul. 4,2006), application submitted on Oct. 21, 2002; (2) B. Z. Jang, et al.“Process for Producing Nano-scaled Graphene Plates,” U.S. patentapplication Ser. No. 10/858,814 (Jun. 3, 2004); and (3) B. Z. Jang, A.Zhamu, and J. Guo, “Process for Producing Nano-scaled Platelets andNanocomposites,” U.S. patent application Ser. No. 11/509,424 (Aug. 25,2006).

NGPs are typically obtained by intercalating natural graphite particleswith a strong acid and/or oxidizing agent to obtain a graphiteintercalation compound (GIC) or graphite oxide (GO), as illustrated inFIG. 1(A) (process flow chart) and FIG. 1(B) (schematic drawing). Thepresence of chemical species or functional groups in the interstitialspaces between graphene planes serves to increase the inter-graphenespacing (d₀₀₂, as determined by X-ray diffraction), therebysignificantly reducing the van der Waals forces that otherwise holdgraphene planes together along the c-axis direction. The GIC or GO ismost often produced by immersing natural graphite powder (20 in FIG.1(A) and 100 in FIG. 1(B)) in a mixture of sulfuric acid, nitric acid(an oxidizing agent), and another oxidizing agent (e.g. potassiumpermanganate or sodium perchlorate). The resulting GIC (22 or 102) isactually some type of graphite oxide (GO) particles. This GIC or GO isthen repeatedly washed and rinsed in water to remove excess acids,resulting in a graphite oxide suspension or dispersion, which containsdiscrete and visually discernible graphite oxide particles dispersed inwater. There are two processing routes to follow after this rinsingstep:

Route 1 involves removing water from the suspension to obtain“expandable graphite,” which is essentially a mass of dried GIC or driedgraphite oxide particles. Upon exposure of expandable graphite to atemperature in the range of typically 800-1,050° C. for approximately 30seconds to 2 minutes, the GIC undergoes a rapid volume expansion by afactor of 30-300 to form “graphite worms” (24 or 104), which are each acollection of exfoliated, but largely un-separated graphite flakes thatremain interconnected. A SEM image of graphite worms is presented inFIG. 2(A).

In Route 1A, these graphite worms (exfoliated graphite or “networks ofinterconnected/non-separated graphite flakes”) can be re-compressed toobtain flexible graphite sheets or foils (26 or 106) that typically havea thickness in the range of 0.1 mm (100 μm)-0.5 mm (500 μm).Alternatively, one may choose to use a low-intensity air mill orshearing machine to simply break up the graphite worms for the purposeof producing the so-called “expanded graphite flakes” (108) whichcontain mostly graphite flakes or platelets thicker than 100 nm (hence,not a nano material by definition).

Exfoliated graphite worms, expanded graphite flakes, and therecompressed mass of graphite worms (commonly referred to as flexiblegraphite sheet or flexible graphite foil) are all 3-D graphiticmaterials that are fundamentally different and patently distinct fromeither the 1-D nano carbon material (CNT or CNF) or the 2-D nano carbonmaterial (graphene sheets or platelets, NGPs). Flexible graphite (FG)foils can be used as a heat spreader material, but exhibiting a maximumin-plane thermal conductivity of typically less than 500 W/mK (moretypically <300 W/mK) and in-plane electrical conductivity no greaterthan 1,500 S/cm. These low conductivity values are a direct result ofthe many defects, wrinkled or folded graphite flakes, interruptions orgaps between graphite flakes, and non-parallel flakes (e.g. SEM image inFIG. 2(B)). Many flakes are inclined with respect to one another at avery large angle (e.g. mis-orientation of 20-40 degrees).

In Route 1B, the exfoliated graphite is subjected to high-intensitymechanical shearing (e.g. using an ultrasonicator, high-shear mixer,high-intensity air jet mill, or high-energy ball mill) to form separatedsingle-layer and multi-layer graphene sheets (collectively called NGPs,33 or 112), as disclosed in our U.S. application Ser. No. 10/858,814.Single-layer graphene can be as thin as 0.34 nm, while multi-layergraphene can have a thickness up to 100 nm, but more typically less than20 nm.

Route 2 entails ultrasonicating the graphite oxide suspension for thepurpose of separating/isolating individual graphene oxide sheets fromgraphite oxide particles. This is based on the notion that theinter-graphene plane separation bas been increased from 0.3354 nm innatural graphite to 0.6-1.1 nm in highly oxidized graphite oxide,significantly weakening the van der Waals forces that hold neighboringplanes together. Ultrasonic power can be sufficient to further separategraphene plane sheets to form separated, isolated, or discrete grapheneoxide (GO) sheets. These graphene oxide sheets can then be chemically orthermally reduced to obtain “reduced graphene oxides” (RGO) typicallyhaving an oxygen content of 0.001%-10% by weight, more typically0.01%-5% by weight, most typically and preferably less than 2% byweight.

For the purpose of defining the claims of the instant application, NGPsinclude discrete sheets/platelets of single-layer and multi-layerpristine graphene, graphene oxide, or reduced graphene oxide (RGO).Pristine graphene has essentially 0% oxygen. RGO typically has an oxygencontent of 0.001%-5% by weight. Graphene oxide (including RGO) can have0.001%-50% by weight of oxygen.

It may be noted that flexible graphite foils (obtained by compressing orroll-pressing exfoliated graphite worms) for electronic device thermalmanagement applications (e.g. as a heat sink material) have thefollowing major deficiencies: (1) As indicated earlier, flexiblegraphite (FG) foils exhibit a relatively low thermal conductivity,typically <500 W/mK and more typically <300 W/mK. By impregnating theexfoliated graphite with a resin, the resulting composite exhibits aneven lower thermal conductivity (typically <<200 W/mK, more typically<100 W/mK). (2) Flexible graphite foils, without a resin impregnatedtherein or coated thereon, are of low strength, low rigidity, and poorstructural integrity. The high tendency for flexible graphite foils toget torn apart makes them difficult to handle in the process of making aheat sink. As a matter of fact, the flexible graphite sheets (typically50-200 μm thick) are so “flexible” that they are not sufficiently rigidto make a fin component material for a finned heat sink. (3) Anothervery subtle, largely ignored or overlooked, but critically importantfeature of FG foils is their high tendency to get flaky with graphiteflakes easily coming off from FG sheet surfaces and emitting out toother parts of a microelectronic device. These highly electricallyconducting flakes (typically 1-200 μm in lateral dimensions and >100 nmin thickness) can cause internal shorting and failure of electronicdevices.

Similarly, solid NGPs (including discrete sheets/platelets of pristinegraphene, GO, and RGO), when packed into a film, membrane, or papersheet (34 or 114) of non-woven aggregates using a paper-making process,typically do not exhibit a high thermal conductivity unless thesesheets/platelets are closely packed and the film/membrane/paper isultra-thin (e.g. <1 μm, which is mechanically weak). This is reported inour earlier U.S. patent application Ser. No. 11/784,606 (Apr. 9, 2007).However, ultra-thin film or paper sheets (<10 μm) are difficult toproduce in mass quantities, and difficult to handle when one tries toincorporate these thin films as a heat sink material. In general, apaper-like structure or mat made from platelets of graphene, GO, or RGO(e.g. those paper sheets prepared by vacuum-assisted filtration process)exhibit many defects, wrinkled or folded graphene sheets, interruptionsor gaps between platelets, and non-parallel platelets (e.g. SEM image inFIG. 3(B)), leading to relatively poor thermal conductivity, lowelectric conductivity, and low structural strength. These papers oraggregates of discrete NGP, GO or RGO platelets alone (without a resinbinder) also have a tendency to get flaky, emitting conductive particlesinto air.

Another prior art graphitic material is the pyrolytic graphite film (notbulk), typically thinner than 100 μm. The lower portion of FIG. 1(A)illustrates a typical process for producing prior art pyrolyticgraphitic films from a polymer. The process begins with carbonizing apolymer film 46 (e.g. polyimide) at a carbonization temperature of400-1,000° C. under a typical pressure of 10-15 Kg/cm² for 2-10 hours toobtain a carbonized material 48, which is followed by a graphitizationtreatment at 2,500-3,200° C. under an ultrahigh pressure of 100-300Kg/cm² for 1-24 hours to form a graphitic film 50. It is technicallyutmost challenging to maintain such an ultrahigh pressure at such anultrahigh temperature. This is a difficult, slow, tedious,energy-intensive, and extremely expensive process. Furthermore, it hasnot been possible to produce pyrolytic graphite film thicker than 100 μmfrom a polymer such as polyimide. This thickness-related problem isinherent to this class of materials due to their difficulty in forminginto a thick film while still maintaining an acceptable degree ofpolymer chain orientation and mechanical strength that are required ofproper carbonization and graphitization.

A second type of pyrolytic graphite is produced by high temperaturedecomposition of hydrocarbon gases in vacuum followed by deposition ofthe carbon atoms to a substrate surface. This vapor phase condensationof cracked hydrocarbons is essentially a chemical vapor deposition (CVD)process. In particular, highly oriented pyrolytic graphite (HOPG) is thematerial produced by subjecting the CVD-deposited pyro-carbon to auniaxial pressure at very high temperatures (typically 3,000-3,300° C.).This entails a thermo-mechanical treatment of combined and concurrentmechanical compression and ultra-high temperature for an extended periodof time in a protective atmosphere; a very expensive, energy-intensive,time-consuming, and technically challenging process. The processrequires ultra-high temperature equipment (with high vacuum, highpressure, or high compression provision) that is not only very expensiveto make but also very expensive and difficult to maintain. Even withsuch extreme processing conditions, the resulting HOPG still possessesmany defects, grain boundaries, and mis-orientations (neighboringgraphene planes not parallel to each other), resulting inless-than-satisfactory in-plane properties. Typically, the best preparedHOPG sheet or block typically contains many poorly aligned grains orcrystals and a vast amount of grain boundaries and defects.

Similarly, the most recently reported graphene thin film (<2 nm)prepared by catalytic CVD of hydrocarbon gas (e.g. C₂H₄) on Ni or Cusurface is not a single-grain crystal, but a poly-crystalline structurewith many grain boundaries and defects. With Ni or Cu being thecatalyst, carbon atoms obtained via decomposition of hydrocarbon gasmolecules at 800-1,000° C. are deposited onto Ni or Cu foil surface toform a sheet of single-layer or few-layer graphene that ispoly-crystalline. The grains are typically much smaller than 100 μm insize and, more typically, smaller than 10 μm in size. These graphenethin films, being optically transparent and electrically conducting, areintended for applications such as the touch screen (to replaceindium-tin oxide or ITO glass) or semiconductor (to replace silicon,Si). Furthermore, the Ni- or Cu-catalyzed CVD process does not lenditself to the deposition of more than 5 graphene planes (typically <2nm) beyond which the underlying Ni or Cu catalyst can no longer provideany catalytic effect. There has been no experimental evidence toindicate that CVD graphene layer thicker than 5 nm is possible.

Thus, it is an object of the present invention to provide a process forproducing graphene oxide (GO)-derived bulk highly oriented graphenestructure, which exhibits a thermal conductivity, electricalconductivity, elastic modulus, and/or tensile strength that iscomparable to or greater than those of the HOPG, CVD graphene film,and/or flexible graphite.

It is another object of the present invention to provide a process forproducing GO-derived bulk highly oriented graphene structure (thickerthan 0.1 mm, preferably thicker than 0.2 mm, more preferably thickerthan 0.5 mm, further more preferably with a thickness in the mm range,and most preferably in the centimeter range) that exhibit a combinationof exceptional thermal conductivity, electrical conductivity, mechanicalstrength, and elastic modulus unmatched by any material of comparablethickness range.

SUMMARY OF THE INVENTION

The present invention provides a process for producing a bulk highlyoriented graphene structure (HOGS) with a thickness greater than 0.1 mm(more typically >0.2 mm, further more typically >0.5 mm, even moretypically >1 mm, still more typically >5 mm, and most typically >10 mm).There is no theoretical limit on the thickness of the HOGS that can beproduced using the presently invented process. This is not a processintended for producing thin films, which are typically defined as havinga thickness less than 100 μm or 0.1 mm. The process includes:

(a) preparing either a graphene oxide dispersion (GO suspension) havinggraphene oxide sheets dispersed in a fluid medium or a GO gels having GOmolecules dissolved in a fluid medium, wherein the GO sheets or GOmolecules contain an oxygen content higher than 5% by weight (typicallyhigher than 10%, more typically higher than 20%, often higher than 30%,and can be up to approximately 50% by weight);

(b) dispensing and depositing the GO dispersion or GO gel onto a surfaceof a supporting solid substrate to form a first layer of graphene oxidehaving a thickness less than 2 mm (preferably less than 1.0 mm, morepreferably less than 0.5 mm, and most preferably less than 0.2 mm),wherein the dispensing and depositing procedure includes subjecting thegraphene oxide dispersion to an orientation-inducing stress;

(c) partially or completely removing the fluid medium from the firstlayer of graphene oxide to form a first dried layer of graphene oxidehaving an inter-plane spacing d₀₀₂ of 0.4 nm to 1.2 nm as determined byX-ray diffraction and an oxygen content no less than 5% by weight;

(d) preparing at least a second layer of dried graphene oxide byrepeating steps (b) and (c) at least one time or simply by slicing thefirst dried layer into multiple pieces of dried graphene oxide;

(e) stacking the first layer of dried graphene oxide with the at leastsecond layer of dried graphene oxide (or stacking multiple pieces ofdried graphene oxide prepared by slicing) under an optional firstcompressive stress to form a mass of multiple layers of graphene oxide;and

(f) heat treating the mass of multiple layers or pieces of driedgraphene oxide under an optional second compressive stress to producethe highly oriented graphene structure at a first heat treatmenttemperature higher than 100° C. to an extent that an inter-plane spacingd₀₀₂ is decreased to a value less than 0.4 nm and the oxygen content isdecreased to less than 5% by weight, wherein said step (f) occursbefore, during, or after said step (e).

In an embodiment, the fluid medium further contains pristine graphenesheets and a pristine graphene to graphene oxide ratio is from 1/100 to100/1.

It may be noted that the step (d) of preparing at least a second driedlayer of graphene oxide may be done by simply slicing the first driedlayer into multiple pieces of dried graphene oxide. The dried layer ofgraphene oxide may be advantageously prepared in a roll-to-roll mannerwith a desired width and length, sufficient to be cut into many pieces.

It may be further noted that step (e) of stacking and step (f) of heattreating may be conducted sequentially or concurrently. In oneembodiment, the process includes (i) heat treating multiple layers orpieces of dried graphene oxide, separately or simultaneously, at a firstheat treatment temperature higher than 100° C. to an extent that aninter-plane spacing d₀₀₂ in any of the dried graphene layer or piece isdecreased to a value less than 0.4 nm and the oxygen content isdecreased to less than 5% by weight; and (ii) stacking multiple layersor pieces of heat-treated graphene oxide under a first compressivestress to form the highly oriented graphene structure. The order ofconducting stacking and heat-treating steps may be reversed.

The orientation-inducing stress may be a shear stress. As an example,the shear stress can be encountered in a situation as simple as a“doctor's blade” that guides the spreading of GO dispersion or GO gelover a plastic or glass surface during a manual casting process. Asanother example, an effective orientation-inducing stress is created inan automated roll-to-roll coating process in which a “knife-on-roll”configuration dispenses GO dispersion or GO gel over a moving solidsubstrate, such as a plastic film. The relative motion between thismoving film and the coating knife acts to effect orientation of graphenesheets along the shear stress direction.

This orientation-inducing stress is a critically important step in theproduction of the presently invented HOGS due to the surprisingobservation that the shear stress enables the GO sheets or GO moleculesto align themselves along a particular direction (e.g. X-direction orlength-direction) or two particular directions (e.g. X- and Y-directionsor length and width directions) to produce preferred orientations.Further surprisingly, these preferred orientations are preserved andoften further enhanced during the subsequent heat treatment of the GOcompact (multiple layers of orientation-controlled GO stacked andcompressed together) to produce the highly oriented graphene structure.Most surprisingly, such preferred orientations are essential to theeventual attainment of exceptionally high thermal conductivity,electrical conductivity, elastic modulus, and tensile or flexuralstrength of the resulting HOGS along a desired direction. These greatproperties in this desired direction could not be obtained without sucha shear stress-induced orientation control.

In an embodiment, the invention provides a process for producing ahighly oriented graphene structure with a thickness no greater than 0.1mm, which process comprises: (a) preparing a pristine graphenedispersion having oxygen-free pristine graphene sheets dispersed in afluid medium; (b) dispensing and depositing the pristine graphenedispersion onto a surface of a supporting substrate to form a layer ofpristine graphene, wherein the dispensing and depositing procedureincludes subjecting the pristine graphene dispersion to anorientation-inducing stress; (c) partially or completely removing thefluid medium from the layer of pristine graphene to form a dried layerof pristine graphene; (d) slicing the dried layer of pristine grapheneinto multiple pieces of dried pristine graphene or repeating steps (b)and (c) to produce multiple layers of dried pristine graphene, andstacking at least two pieces or two layers of dried pristine grapheneunder an optional first compressive stress to form a mass of multiplepieces of pristine graphene; and (e) heat-treating the mass of multiplepieces or layers of dried pristine graphene at a first heat treatmenttemperature higher than 2,000° C. under a second compressive stress fora sufficient period of time to produce the highly oriented graphenestructure.

During the coating or casting procedure in all versions of the presentlyinvented process, the thickness of the coated or cast films (layers)cannot be too high, otherwise a high degree of GO or graphene sheetorientation cannot be achieved. In general, the coated or cast films(wet layers) must be sufficiently thin that when they become dried, theyform a dried layer of graphene oxide having a thickness no greater than100 μm, preferably no greater than 50 μm, more preferably no greaterthan 30 μm, further preferably no greater than 20 μm, and mostpreferably no greater than 10 μm. Through extensive and in-depthexperimental studies we have come to unexpectedly realize that thethinner these dried orientation-controlled GO layers, the higher thedegree of GO sheet orientation and the higher in-plane thermalconductivity and electrical conductivity of the resulting HOGS. A highdegree of GO plane orientation cannot be achieved if one simply cast avery thick mass of GO suspension or GO gel, drying the mass, compactingthe mass (using a uniaxial compressive stress), and heat-treating thecompact. The resulting graphitic structure, not a HOGS, does not havethe desired degree of graphene plane orientation and its thermalconductivity and electrical conductivity are no better than those of astructure prepared by stacking and compressing multiple sheets offlexible graphite foil.

In an embodiment, step (f) further includes heat-treating the grapheneoxide mass at a second heat treatment temperature higher than 280° C.for a length of time sufficient for decreasing an inter-plane spacingd₀₀₂ to a value of from 0.3354 nm to 0.36 nm and decreasing the oxygencontent to less than 2% by weight.

In an embodiment, the fluid medium consists of water and/or an alcohol.It is highly advantageous that the fluid medium does not contain anyobnoxious chemical.

In a preferred embodiment, the second (or final) heat treatmenttemperature includes at least a temperature selected from (A) 300-1,500°C., (B) 1,500-2,100° C., and/or (C) higher than 2,100° C. Preferably,the second heat treatment temperature includes a temperature in therange of 300-1,500° C. for at least 1 hour and then a temperature in therange of 1,500-3,200° C. for at least another hour.

The process typically involves at least 3 layers of dried graphene oxidetogether and more typically a large number of layers or pieces of driedgraphene oxide, preferably under a compressive stress. There is nolimitation on the number of layers that can be stacked and thenchemically linked together to become a single integrated entity (notjust an aggregate of discrete graphene sheets). This is trulyunexpected.

In one embodiment, the graphene oxide dispersion has at least 1%(preferably at least 3% and often greater than 5%) by weight of grapheneoxide dispersed in the fluid medium. When the weight percentage of GOsheets exceeds at least 5% by weight, these sheets appear to form aliquid crystal phase.

The graphene oxide dispersion or GO gel is prepared by immersing agraphitic material in a powder or fibrous form in an oxidizing liquid ina reaction vessel at a reaction temperature for a length of timesufficient to obtain said graphene oxide dispersion wherein saidgraphitic material is selected from natural graphite, artificialgraphite, meso-phase carbon, meso-phase pitch, meso-carbon micro-bead,soft carbon, hard carbon, coke, carbon fiber, carbon nano-fiber, carbonnano-tube, or a combination thereof and wherein said graphene oxide hasan oxygen content no less than 5% by weight.

In a preferred embodiment, steps (b) and (c) include feeding a sheet ofa solid substrate material from a roller to a deposition zone,depositing a layer of GO dispersion or GO gel onto a surface of thesheet of solid substrate material to form the graphene oxide layerthereon, drying the GO dispersion or GO gel to form the dried grapheneoxide layer deposited on the substrate surface, and collecting the driedgraphene oxide layer-deposited substrate sheet on a collector roller.This is a roll-to-roll or reel-to-reel process that can be conducted ona continuous basis.

In one embodiment, wherein the first and/or second heat treatmenttemperature contains a temperature in the range of 500° C.-1,500° C.,the resulting highly oriented graphene structure has an oxygen contentless than 1%, an inter-graphene spacing less than 0.345 nm, a thermalconductivity of at least 1,000 W/mK, and/or an electrical conductivityno less than 3,000 S/cm.

In another embodiment, wherein the first and/or second heat treatmenttemperature contains a temperature in the range of 1,500° C.-2,100° C.,the highly oriented graphene structure has an oxygen content less than0.01%, an inter-graphene spacing less than 0.337 nm, a thermalconductivity of at least 1,300 W/mK, and/or an electrical conductivityno less than 5,000 S/cm.

In a preferred embodiment, wherein the first and/or second heattreatment temperature contains a temperature greater than 2,100° C., thehighly oriented graphene structure has an oxygen content no greater than0.001%, an inter-graphene spacing less than 0.336 nm, a mosaic spreadvalue no greater than 0.7, a thermal conductivity of at least 1,500W/mK, and/or an electrical conductivity no less than 10,000 S/cm.

In another preferred embodiment, wherein the first and/or second heattreatment temperature contains a temperature no less than 2,500° C., thehighly oriented graphene structure has an inter-graphene spacing lessthan 0.336 nm, a mosaic spread value no greater than 0.4, a thermalconductivity greater than 1,600 W/mK, and/or an electrical conductivitygreater than 10,000 S/cm.

Typically, the highly oriented graphene structure exhibits aninter-graphene spacing less than 0.337 nm and a mosaic spread value lessthan 1.0. More typically, the highly oriented graphene structureexhibits a degree of graphitization no less than 80% (preferably andmore typically no less than 90%) and/or a mosaic spread value less than0.4.

Due to the notion that highly aligned GO sheets or GO molecules can bechemically merged together in an edge-to-edge manner, the resultinghighly oriented graphene structure has a grain size that issignificantly larger than the maximum grain size of the startinggraphitic material prior to or during oxidation of the graphiticmaterial. In other words, if the graphene oxide dispersion is obtainedfrom a graphitic material having a maximum original graphite grain size,then the resulting highly oriented graphene structure is normally asingle crystal or a poly-crystal graphene structure having a grain sizelarger than this maximum original grain size.

Internal structure-wise, the highly oriented graphene structure containschemically bonded graphene planes that are parallel to one another. Thegraphene oxide dispersion is typically obtained from a graphiticmaterial having multiple graphite crystallites exhibiting no preferredcrystalline orientation as determined by an X-ray diffraction orelectron diffraction method. However, the highly oriented graphenestructure is typically a single crystal or a poly-crystal graphenestructure having a preferred crystalline orientation as determined bysaid X-ray diffraction or electron diffraction method. In some cases,the highly oriented graphene structure contains a combination of sp² andsp³ electronic configurations. In the invented process, the step ofheat-treating induces chemical linking, merging, or chemical bonding ofgraphene oxide molecules, and/or re-graphitization or re-organization ofa graphitic structure.

The invented process typically results in a highly oriented graphenestructure that has an electrical conductivity greater than 5,000 S/cm, athermal conductivity greater than 800 W/mK, a physical density greaterthan 1.9 g/cm3, a flexural strength greater than 100 MPa, and/or anelastic modulus greater than 60 GPa. More typically, the highly orientedgraphene structure has an electrical conductivity greater than 8,000S/cm, a thermal conductivity greater than 1,200 W/mK, a physical densitygreater than 2.0 g/cm3, a flexural strength greater than 140 MPa, and/oran elastic modulus greater than 80 GPa. Still more typically, the highlyoriented graphene structure has an electrical conductivity greater than12,000 S/cm, a thermal conductivity greater than 1,500 W/mK, a physicaldensity greater than 2.1 g/cm³, a flexural strength greater than 160MPa, and/or an elastic modulus greater than 120 GPa.

The present invention also provides a highly oriented graphene structureproduced by using any version of the presently invented process. Thepresent invention further provides a thermal management device thatcontains a highly oriented graphene structure produced by using anyversion of the presently invented process.

This new class of materials (i.e., a GO-derived highly oriented graphenestructure) has the following characteristics (separately or incombination) that distinguish themselves from HOPG, stacked flexiblegraphite sheets, and stacked paper/film/membrane sheets of discretegraphene/GO/RGO sheets/platelets:

(1) This HOGS is an integrated graphene entity that is either a graphenesingle crystal (single grain only) or a poly-crystal (multiple grainswith exceptionally large grain sizes). The highly oriented graphenestructure has all the graphene planes in all the grains beingessentially oriented parallel to one another (i.e., the crystallographicc-axis of all grains essentially pointing in an identical direction).

(2) The HOGS is an integrated graphene entity that is not a simpleaggregate or stack of multiple discrete graphite flakes or discreteplatelets of graphene/GO/RGO, and does not contain any discernible ordiscrete flake/platelet derived from the original GO sheets. Theseoriginally discrete flakes or platelets have been chemically bonded orlinked together to form larger grains (grain size being larger than theoriginal platelet/flake size).

(3) This HOGS is not made by using a binder or adhesive to glue discreteflakes or platelets together. Instead, under select heat treatmentconditions, well-aligned GO sheets or GO molecules are capable ofchemically merging with one another mainly in an edge-to-edge manner toform giant 2-D graphene grains, but possibly also with adjacent GOsheets below or above to form 3-D networks of graphene chains Throughjoining or forming of covalent bonds with one another, the GO sheets areadhered into an integrated graphene entity, without using any externallyadded linker or binder molecules or polymers.

(4) This HOGS, a single crystal or poly-crystal with essentially allgraphene planes having an identical crystallographic c-axis, is derivedfrom GO, which is in turn obtained from moderate or heavy oxidation ofnatural graphite or artificial graphite particles each originally havingmultiple graphite crystallites that are randomly oriented. Prior tobeing chemically oxidized to become GO dispersion (moderate-to-heavyoxidation of graphite) or GO gel (heavy oxidation for a sufficientlylong oxidation time to achieve fully separated GO molecules dissolved inwater or other polar liquid), these starting or original graphitecrystallites have an initial length (L_(a) in the crystallographica-axis direction), initial width (L_(b) in the b-axis direction), andthickness (L_(c) in the c-axis direction). The resulting HOGS typicallyhas a length or width significantly greater than the L_(a) and L_(b) ofthe original graphite crystallites.

(5) This process involves significantly lower heat treatmenttemperatures and lower pressure as compared with the processes forproducing HOPG from either carbonized polymers (e.g. polyimide) or theCVD graphite. The presently invented process is simpler (hence, morereliable), faster, less energy-intensive, and highly scalable.

(6) This process for producing a bulk GO-derived HOGS can be conductedon a continuous roll-to-roll basis and, hence, is much morecost-effective. No other process is known to be capable of producingHOPG structures on a continuous basis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 (A) A flow chart illustrating various prior art processes ofproducing exfoliated graphite products (flexible graphite foils andflexible graphite composites) and pyrolytic graphite (bottom portion),along with a process for producing graphene oxide gel or GO dispersion21, oriented GO layer 35, layers of dried GO 37 a, GO compact 37 bobtained by stacking and compressing multiple layers or pieces of driedGO, and HOGS 37 c; (B) Schematic drawing illustrating the processes forproducing conventional paper, mat, film, and membrane of simplyaggregated graphite or NGP flakes/platelets. All processes begin withintercalation and/or oxidation treatment of graphitic materials (e.g.natural graphite particles).

FIG. 2 (A) A SEM image of a graphite worm sample after thermalexfoliation of graphite intercalation compounds (GICs) or graphite oxidepowders; (B) An SEM-image of a cross-section of a flexible graphitefoil, showing many graphite flakes with orientations not parallel to theflexible graphite foil surface and also showing many defects, kinked orfolded flakes.

FIG. 3 (A) A SEM image of a GO-derived HOGS, wherein multiple grapheneplanes (having an initial length/width of 30 nm-300 nm in originalgraphite particles) have been oxidized, exfoliated, re-oriented, andseamlessly merged into continuous-length graphene sheets or layers thatcan run for tens of centimeters wide or long (only a 50 μm width of a10-cm wide HOGS being shown in this SEM image); (B) A SEM image of across-section of a conventional graphene paper/film prepared fromdiscrete graphene sheets/platelets using a paper-making process (e.g.vacuum-assisted filtration). The image shows many discrete graphenesheets being folded or interrupted (not integrated), with orientationsnot parallel to the film/paper surface and having many defects orimperfections; (C) Schematic drawing and an attendant SEM image toillustrate the formation process of a HOGS that is composed of multiplegraphene planes that are parallel to one another and are chemicallybonded in the thickness-direction or crystallographic c-axis direction;and (D) One plausible chemical linking mechanism (only 2 GO moleculesare shown as an example; a large number of GO molecules can bechemically linked together to form a graphene layer).

FIG. 4 (A) Thermal conductivity values of the GO dispersion(suspension)-derived HOGS, GO gel-derived HOGS, stacked and compressedsheets of GO platelet paper, and stacked and compressed sheets of FGfoil plotted as a function of the final heat treatment temperature forgraphitization or re-graphitization; (B) Thermal conductivity values ofthe GO dispersion-derived HOGS, the CVD carbon- and thepolyimide-derived HOPG, all plotted as a function of the finalgraphitization or re-graphitization temperature; and (C) Electricconductivity values.

FIG. 5 X-ray diffraction curves of (A) a GO layer, (B) GO layerthermally reduced at 150° C. (partially reduced), (C) reduced andre-graphitized bulk HOGS, (D) highly re-graphitized and re-crystallizedHOGS showing a high-intensity (004) peak, and (E) a polyimide-derivedHOPG with a HTT as high as 3,000° C.

FIG. 6 (A) Inter-graphene plane spacing measured by X-ray diffraction;(B) the oxygen content in the GO suspension-derived HOGS; (C)correlation between inter-graphene spacing and the oxygen content; and(D) thermal conductivity of GO dispersion-derived HOGS, GO gel-derivedHOGS, and stacked sheets of flexible graphite (FG) foil, all plotted asa function of the final heat treatment temperature.

FIG. 7 (A) Flexural strength and (B) flexural modulus of the HOGF fromGO dispersion, HOGS from GO gel, stacked and compressed pieces of GOplatelet paper, and stacked and compressed pieces of flexible graphitefoil sheets over a range of heat treatment temperatures.

FIG. 8 Thermal conductivity of HOGS samples (prepared with a final heattreatment temperature of 1,500° C. and a final thickness ofapproximately 0.3 mm) plotted as a function of the thickness value ofthe individual GO layers:

FIG. 9 Thermal conductivity of HOGS samples (prepared with a final heattreatment temperature of 1,000° C. and a final thickness ofapproximately 0.5 mm) plotted as a function of the proportion ofpristine graphene sheets in a GO suspension.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a process for producing a bulk highlyoriented graphene structure with a thickness greater than 0.2 mm (moretypically >0.5 mm, even more typically >1 mm, further more typically >5mm, and most typically >10 mm). The process includes:

(a) preparing either a graphene oxide (GO) dispersion having grapheneoxide sheets dispersed in a fluid medium or a GO gel having GO moleculesdissolved in a fluid medium, wherein the GO sheets or GO moleculescontain an oxygen content higher than 5% by weight (typically higherthan 10%, more typically higher than 20%, often higher than 30%, and canbe up to approximately 50% by weight). Graphene oxide (GO) sheets arepreferably single-layer or few-layer graphene sheets (up to 10 layers ofgraphene planes of carbon atoms) having edge- and surface-borneoxygen-containing functional groups. These O-containing functionalgroups enable good dispersion or dissolution of GO sheets in a moreenvironmentally benign fluid medium, such as water and/or alcohol(methanol, ethanol, propanol, etc.).(b) dispensing and depositing the GO dispersion or GO gel onto a surfaceof a supporting solid substrate to form a first layer of graphene oxidehaving a thickness less than 2 mm (preferably less than 1 mm, morepreferably less than 0.5 mm, and most preferably less than 0.2 mm),wherein the dispensing and depositing procedure include subjecting thegraphene oxide dispersion or GO gel to an orientation-inducing stress;(c) partially or completely removing the fluid medium from the firstlayer of graphene oxide to form a first layer of dried graphene oxidehaving an inter-plane spacing d₀₀₂ of 0.4 nm to 1.2 nm as determined byX-ray diffraction and an oxygen content no less than 5% by weight;(d) preparing at least a second layer of dried graphene oxide byrepeating steps (b) and (c) at least one time or simply by slicing thefirst dried layer into multiple pieces of dried graphene oxide;(e) stacking the first layer of dried graphene oxide with the at leastsecond layer of dried graphene oxide (or multiple pieces of driedgraphene oxide prepared by slicing) under an optional first compressivestress to form a mass of multiple layers of dried graphene oxide; and(f) heat treating the mass of multiple layers or pieces of driedgraphene oxide under an optional second compressive stress to producethe highly oriented graphene structure at a first heat treatmenttemperature higher than 100° C. to an extent that an inter-plane spacingd₀₀₂ is decreased to a value less than 0.4 nm and the oxygen content isdecreased to less than 5% by weight, wherein said step (f) occursbefore, during, or after said step (e).

In an embodiment, step (f) further includes heat-treating the grapheneoxide mass at a second heat treatment temperature higher than the firstheat treatment temperature and higher than 280° C. for a length of timesufficient for decreasing an inter-plane spacing d₀₀₂ to a value of from0.3354 nm to 0.36 nm and decreasing the oxygen content to less than 2%by weight (most preferably between 0.001% to 0.01% by weight).

In a preferred embodiment, the second (or final) heat treatmenttemperature includes at least a temperature selected from (A) 100-300°C., (B) 300-1,500° C., (C) 1,500-2,500° C., and/or (D) higher than2,500° C. Preferably, the second heat treatment temperature includes atemperature in the range of 300-1,500° C. for at least 1 hour and then atemperature in the range of 1,500-3,200° C. for at least another hour.

The highly oriented graphene structure contains chemically bonded andmerged graphene planes. These planar aromatic molecules or grapheneplanes (hexagonal structured carbon atoms having a small amount ofoxygen-containing group) are parallel to one another. The lateraldimensions (length or width) of these planes are huge, typically severaltimes or even orders of magnitude larger than the maximum crystallitedimension (or maximum constituent graphene plane dimension) of thestarting graphite particles. The presently invented bulk highly orientedgraphene structure is a “giant graphene crystal” or “giant grapheneparticle” having all constituent graphene planes being essentiallyparallel to one another. This is a unique and new class of material thathas not been previously discovered, developed, or suggested to possiblyexist.

The oriented graphene oxide layer is itself a very unique and novelclass of material that surprisingly has great cohesion power(self-bonding, self-polymerizing, and self-crosslinking capability).These characteristics have not been taught or hinted in the prior art.The GO is obtained by immersing powders or filaments of a startinggraphitic material in an oxidizing liquid medium (e.g. a mixture ofsulfuric acid, nitric acid, and potassium permanganate) in a reactionvessel. The starting graphitic material may be selected from naturalgraphite, artificial graphite, meso-phase carbon, meso-phase pitch,meso-carbon micro-bead, soft carbon, hard carbon, coke, carbon fiber,carbon nano-fiber, carbon nano-tube, or a combination thereof.

When the starting graphite powders or filaments are mixed in theoxidizing liquid medium, the resulting slurry is a heterogeneoussuspension and appears dark and opaque. When the oxidation of graphiteproceeds at a reaction temperature for a sufficient length of time, thereacting mass can eventually become a suspension that appears slightlygreen and yellowish, but remain opaque. If the degree of oxidation issufficiently high (e.g. having an oxygen content between 20% and 50% byweight, preferably between 30% and 50%) and all the original grapheneplanes are fully oxidized, exfoliated and separated to the extent thateach oxidized graphene plane (now a graphene oxide sheet or molecule) issurrounded by the molecules of the liquid medium, one obtains a GO gel.The GO gel is optically translucent and is essentially a homogeneoussolution, as opposed to a heterogeneous suspension.

This GO suspension or GO gel typically contains some excess amount ofacids and can be advantageously subjected to some acid dilutiontreatment to increase the pH value (preferably >4.0). The GO suspension(dispersion) preferably contain at least 1% by weight of GO sheetsdispersed in a liquid medium, more preferably at least 3% by weight, andmost preferably at least 5% by weight. It is advantageous to have anamount of GO sheets sufficient for forming a liquid crystalline phase.We have surprisingly observed that GO sheets in a liquid crystal statehave the highest tendency to get readily oriented under the influence ofa shear stress created by a commonly used casting or coating process.

In step (b), the GO suspension is formed into a GO layer preferablyunder the influence of a shear stress. One example of such a shearingprocedure is casting or coating a thin film of GO suspension or GO gelusing a coating machine. This procedure is similar to a layer ofvarnish, paint, coating, or ink being coated onto a solid substrate. Theroller, “doctor's blade”, or wiper creates a shear stress when the filmis shaped, or when there is a relative motion between theroller/blade/wiper and the supporting substrate. Quite unexpectedly andsignificantly, such a shearing action enables the planar GO sheets or GOmolecules to well align along, for instance, a shearing direction.Further surprisingly, such a molecular alignment state or preferredorientation is not disrupted when the liquid components in the GOsuspension or GO gel are subsequently removed to form a well-packedlayer of highly aligned GO sheets that are at least partially dried. Thedried GO mass has a high birefringence coefficient between an in-planedirection and the normal-to-plane direction.

In an embodiment, the above-described procedure is repeated to produceanother layer of dried GO whose constituent GO sheets are alsowell-aligned. In an alternative embodiment, a layer of dried GO issliced (slit) into multiple pieces of dried GO of comparable dimensions.Subsequently, there are two main routes of thermal-mechanical proceduresto follow: In a first route, multiple pieces or layers of dried GO arethen stacked together to form a stacked structure, which is compressedto form a GO compact. This compact is then subjected to a heat treatmentto produce a heat-treated compact that involves at least a first(initial) heat treatment temperature greater than 80° C., preferablygreater than 100° C., more preferably greater than 280-300° C., furthermore preferably greater than 500° C. and can be as high as 1,500° C.

In a second route, individual layers or pieces of dried GO are treatedat a first heat treatment temperature for a desired length of time, andmultiple layers or pieces of heat-treated GO are then compressed to forma heat-treated compact. In either the first or the second route, theheat-treated compact is then subjected to a further heat treatment thatinvolves at least a second temperature that is significantly higher thanthe first heat treatment temperature.

A properly programmed heat treatment procedure can involve just a singleheat treatment temperature (e.g. a first heat treatment temperatureonly), at least two heat treatment temperatures (first temperature for aperiod of time and then raised to a second temperature and maintained atthis second temperature for another period of time), or any othercombination of heat treatment temperatures that involve an initialtreatment temperature (first temperature) and a final heat treatmenttemperature, higher than the first. The highest or final heat treatmenttemperature that the GO mass experiences may be divided into fourdistinct heat treatment temperature (HIT) regimes:

-   Regime 1 (up to 300° C.): In this temperature range (the thermal    reduction regime), a GO mass (an individual GO layer or a    pre-stacked and compressed compact of multiple GO layers) primarily    undergoes thermally-induced reduction reactions, leading to a    reduction of oxygen content from typically 20-50% (as dried) to    approximately 5-6%. This treatment results in a reduction of    inter-graphene spacing from approximately 0.6-1.2 nm (as dried) down    to approximately 0.4 nm, and an increase in in-plane thermal    conductivity from approximately 100 W/mK to 450 W/mK. Even with such    a low temperature range, some chemical linking occurs. The GO    molecules remain well-aligned, but the inter-GO spacing remains    relatively large (0.4 nm or larger). Many O-containing functional    groups survive.

Regime 2 (300° C.-1,500° C.): In this chemical linking regime, extensivechemical combination, polymerization, and cross-linking between adjacentGO sheets or GO molecules occur. The oxygen content is reduced totypically 0.7% (<<1%) after chemical linking, resulting in a reductionof inter-graphene spacing to approximately 0.345 nm. This implies thatsome initial graphitization has already begun at such a low temperature,in stark contrast to conventional graphitizable materials (such ascarbonized polyimide film) that typically require a temperature as highas 2,500° C. to initiate graphitization. This is another distinctfeature of the presently invented HOGS and its production processes.These chemical linking reactions result in an increase in in-planethermal conductivity to 800-1,200 W/mK, and/or in-plane electricalconductivity to 3,000-4,000 S/cm.

-   Regime 3 (1,500-2,500° C.): In this ordering and re-graphitization    regime, extensive graphitization or graphene plane merging occurs,    leading to significantly improved degree of structural ordering. As    a result, the oxygen content is reduced to typically 0.01% and the    inter-graphene spacing to approximately 0.337 nm (achieving degree    of graphitization from 1% to approximately 80%, depending upon the    actual HTT and length of time). The improved degree of ordering is    also reflected by an increase in in-plane thermal conductivity    to >1,200-1,500 W/mK, and/or in-plane electrical conductivity to    5,000-7,000 S/cm.-   Regime 4 (higher than 2,500° C.): In this re-crystallization and    perfection regime, extensive movement and elimination of grain    boundaries and other defects occur, resulting in the formation of    nearly perfect single crystals or poly-crystalline graphene crystals    with huge grains, which can be orders of magnitude larger than the    original grain sizes of the starting graphite particles for the    production of GO suspension. The oxygen content is essentially    eliminated, typically 0%-0.001%. The inter-graphene spacing is    reduced to down to approximately 0.3354 nm (degree of graphitization    from 80% to nearly 100%), corresponding to that of a perfect    graphite single crystal. Quite interestingly, the graphene    poly-crystal has all the graphene planes being closely packed and    bonded, and all the planes are aligned along one direction, a    perfect orientation. Such a perfectly oriented structure has not    been produced even with the HOPG that was produced by subjecting    pyrolytic graphite concurrently to an ultra-high temperature (3,400°    C.) under an ultra-high pressure (300 Kg/cm²). The highly oriented    graphene structure can achieve such a highest degree of perfection    with a significantly lower temperature and an ambient (or slightly    higher compression) pressure. The structure thus obtained exhibits    an in-plane thermal conductivity up to slightly >1,700 W/mK, and    in-plane electrical conductivity to a range from 15,000 to 20,000    S/cm.    The presently invented highly oriented graphene structure can be    obtained by heat-treating the stacked multiple layers of dried GO    with a temperature program that covers at least the first regime    (typically requiring 1-4 hours in this temperature range if the    temperature never exceeds 500° C.), more commonly covers the first    two regimes (1-2 hours preferred), still more commonly the first    three regimes (preferably 0.5-2.0 hours in Regime 3), and most    commonly all the 4 regimes (Regime 4, for 0.2 to 1 hour, may be    implemented to achieve the highest conductivity).

X-ray diffraction patterns were obtained with an X-ray diffractometerequipped with CuKcv radiation. The shift and broadening of diffractionpeaks were calibrated using a silicon powder standard. The degree ofgraphitization, g, was calculated from the X-ray pattern using theMering's Eq, d₀₀₂=0.3354 g+0.344 (1−g), where d₀₀₂ is the interlayerspacing of graphite or graphene crystal in nm. This equation is validonly when d₀₀₂ is equal or less than approximately 0.3440 nm. The HOGShaving a d₀₀₂ higher than 0.3440 nm reflects the presence ofoxygen-containing functional groups (such as —OH, >O, and —COOH ongraphene molecular plane surfaces) that act as a spacer to increase theinter-graphene spacing.

Another structural index that can be used to characterize the degree ofordering of the presently invented HOGS and conventional graphitecrystals is the “mosaic spread,” which is expressed by the full width athalf maximum of a rocking curve (X-ray diffraction intensity) of the(002) or (004) reflection. This degree of ordering characterizes thegraphite or graphene crystal size (or grain size), amounts of grainboundaries and other defects, and the degree of preferred grainorientation. A nearly perfect single crystal of graphite ischaracterized by having a mosaic spread value of 0.2-0.4. Most of ourHOGS samples have a mosaic spread value in this range of 0.2-0.4 (ifproduced with a heat treatment temperature (HTT) no less than 2,500°C.). However, some values are in the range of 0.4-0.7 if the HTT isbetween 1,500 and 2,500° C., and in the range of 0.7-1.0 if the HTT isbetween 300 and 1,500° C.

The graphene oxide suspension may be prepared by immersing a graphiticmaterial (in a powder or fibrous form) in an oxidizing liquid to form areacting slurry in a reaction vessel at a reaction temperature for alength of time sufficient to obtain graphene oxide (GO) sheets dispersedin a residual liquid. Typically, this residual liquid is a mixture ofacid (e.g. sulfuric acid) and oxidizer (e.g. potassium permanganate orhydrogen peroxide). This residual liquid is then washed and replacedwith water and/or alcohol to produce a GO dispersion wherein discrete GOsheets (single-layer or multi-layer GO) are dispersed in the fluid. Thedispersion is a heterogeneous suspension of discrete GO sheets suspendedin a liquid medium and it looks optically opaque and dark (relativelylow degree of oxidation) or slightly green and yellowish (if the degreeof oxidation is high).

Now, if the GO sheets contain a sufficient amount of oxygen-containingfunctional groups and the resulting dispersion (suspension or slurry) ismechanically sheared or ultrasonicated to produce individual GO sheetsor molecules that are dissolved (not just dispersed) in water and/oralcohol or other polar solvent, we can reach a material state called “GOgel” in which all individual GO molecules are surrounded by themolecules of the liquid medium. The GO gel looks like a homogeneoussolution which is translucent and no discernible discrete GO or graphenesheets can be visibly identified. Useful starting graphitic materialsinclude natural graphite, artificial graphite, meso-phase carbon,meso-phase pitch, meso-carbon micro-bead, soft carbon, hard carbon,coke, carbon fiber, carbon nano-fiber, carbon nano-tube, or acombination thereof. As the oxidizing reaction proceeds to a criticalextent and individual GO sheets are fully separated (now with grapheneplane and edges being heavily decorated with oxygen-containing groups),an optically transparent or translucent solution is formed, which is theGO gel.

Preferably, the GO sheets in such a GO dispersion or the GO molecules insuch a GO gel are in the amount of 1%-15% by weight, but can be higheror lower. More preferably, the GO sheets are 2%-10% by weight in thesuspension. Most preferably, the amount of GO sheets is sufficient toform a liquid crystal phase in the dispersing liquid. The GO sheets havean oxygen content typically in the range from 5% to 50% by weight, moretypically from 10% to 50%, and most typically from 20% to 46% by weight.

The graphene oxide suspension- or GO gel-derived bulk (non-thin film)highly oriented graphene structure (HOGS) has the followingcharacteristics:

-   (1) The bulk HOGS (>>100 μm, and more typically >>200 μm in    thickness) is an integrated graphene oxide or oxygen-free graphene    structure that is typically a poly-crystal having large grains. The    HOGS has wide/long chemically bonded graphene planes that are all    essentially oriented parallel to one another. In other words, the    crystallographic c-axis directions of all the constituent graphene    planes in all grains are essentially pointing in the same direction.-   (2) The HOGS is a fully integrated, essentially void-free, single    graphene entity or monolith containing no discrete flakes or    platelets derived from the GO suspension. In contrast, the    paper-like sheets of exfoliated graphite worms (i.e., flexible    graphite foils), mats of expanded graphite flakes (>100 nm in    thickness), and paper or membrane of graphene or GO platelets (<100    nm) are a simple, un-bonded aggregate/stack of multiple discrete    graphite flakes or discrete platelets of graphene, GO, or RGO. The    flakes or platelets in these paper/membrane/mats are poorly oriented    and have lots of kinks, bends, and wrinkles. Many voids or other    defects are present in these paper/membrane/mats.-   (3) In prior art processes, discrete graphene sheets (<<100 nm,    typically <10 nm) or expanded graphite flakes (>100 nm) that    constitute the original structure of graphite particles could be    obtained via expanding, exfoliating, and separating treatments. By    simply mixing and re-compressing these discrete sheets/flakes into a    bulk object, one could attempt to orient these sheets/flakes    hopefully along one direction through compression. However, with    these conventional processes, the constituent flakes or sheets of    the resulting aggregate would remain as discrete    flakes/sheets/platelets that can be easily discerned or clearly    observed even with an un-assisted eye or under a low-magnification    optical microscope (×100-×1000).

In contrast, the preparation of the presently invented HOGS involvesheavily oxidizing the original graphite particles, to the extent thatpractically every one of the original graphene planes has been oxidizedand isolated from one another to become individual molecules thatpossess highly reactive functional groups (e.g. —OH, >O, and —COOH) atthe edge and, mostly, on graphene planes as well. These individualhydrocarbon molecules (containing elements such as O and H, in additionto carbon atoms) are dispersed in a liquid medium (e.g. mixture of waterand alcohol) to form a GO dispersion. This dispersion is then cast orcoated onto a smooth substrate surface, typically under shear stressfield conditions, and the liquid components are then removed to form adried GO layer. Multiple layers of the dried GO are stacked andcompacted together to form a GO bulk. When heated, these highly reactivemolecules react and chemically join with one another mostly in lateraldirections along graphene planes (in an edge-to-edge manner) and, insome cases, between graphene planes as well.

Illustrated in FIG. 3( d) is a plausible chemical linking mechanismwhere only 2 aligned GO molecules are shown as an example, although alarge number of GO molecules can be chemically linked together to form aHOGS. Further, chemical linking could also occur face-to-face, not justedge-to-edge. These linking and merging reactions proceed in such amanner that the molecules are chemically merged, linked, and integratedinto one single entity. The molecules (GO sheets) completely lose theirown original identity and they no longer are discretesheets/platelets/flakes. There is only one single layer-like structurethat is essentially a network of interconnected giant molecules with anessentially infinite molecular weight. This may also be described as agraphene poly-crystal (with several grains, but typically nodiscernible, well-defined grain boundaries). All the constituentgraphene planes are very large in lateral dimensions (length and width)and, if constituent layers of dried GO are stacked, compacted, andheat-treated at a higher temperature (e.g. >1,500° C. or much higher),these graphene planes are essentially bonded together with one anotherand aligned parallel to one another.

In-depth studies using a combination of SEM, TEM, selected areadiffraction, X-ray diffraction, AFM, Raman spectroscopy, and FTIRindicate that the HOGO is composed of several huge graphene planes (withlength/width typically >>100 μm, more typically >>1 mm, and mosttypically >>1 cm). These giant graphene planes are stacked and bondedalong the thickness direction (crystallographic c-axis direction) oftenthrough not just the van der Waals forces (as in conventional graphitecrystallites), but also covalent bonds, if the final heat treatmenttemperature is lower than 2,500° C. In these cases, wishing not to belimited by theory, but Raman and FTIR spectroscopy studies appear toindicate the co-existence of sp² (dominating) and sp^(a) (weak butexisting) electronic configurations, not just the conventional sp² ingraphite.

-   (4) This HOGS is not made by gluing or bonding discrete    flakes/platelets together with a resin binder, linker, or adhesive.    Instead, GO sheets (molecules) in the GO dispersion or GO gel are    merged through joining or forming of covalent bonds with one    another, into an integrated graphene entity, without using any    externally added linker or binder molecules or polymers.-   (5) This HOGS is typically a poly-crystal composed of large grains    having incomplete grain boundaries, typically with the    crystallographic c-axis in all grains being essentially parallel to    each other. This entity is derived from a GO suspension or GO gel,    which is in turn obtained from natural graphite or artificial    graphite particles originally having multiple graphite crystallites.    Prior to being chemically oxidized, these starting graphite    crystallites have an initial length (L_(a) in the crystallographic    a-axis direction), initial width (L_(b) in the b-axis direction),    and thickness (L_(c) in the c-axis direction). Upon heavy oxidation,    these initially discrete graphite particles are chemically    transformed into highly aromatic graphene oxide molecules having a    significant concentration of edge- or surface-borne functional    groups (e.g. —OH, —COOH, etc.). These aromatic GO molecules in the    GO suspension have lost their original identity of being part of a    graphite particle or flake. Upon removal of the liquid component    from the suspension, the resulting GO molecules form an essentially    amorphous structure. Upon heat treatments, these GO molecules are    chemically merged and linked into a unitary or monolithic graphene    entity that is highly ordered.

The resulting unitary graphene entity typically has a length or widthsignificantly greater than the L_(a) and L_(b) of the originalcrystallites. The length/width of this HOGS is significantly greaterthan the L_(a) and L_(b) of the original crystallites. Even theindividual grains in a poly-crystalline HOGS have a length or widthsignificantly greater than the L_(a) and L_(b) of the originalcrystallites. They can be as large as the length or width of the HOGSitself, not just 2 or 3 times higher than the initial L_(a) and L_(b) ofthe original crystallites.

-   (6) Due to these unique chemical composition (including oxygen    content), morphology, crystal structure (including inter-graphene    spacing), and structural features (e.g. high degree of orientations,    few defects, incomplete grain boundaries, chemical bonding and no    gap between graphene sheets, and no interruptions in graphene    planes), the graphene oxide-derived HOGS has a unique combination of    outstanding thermal conductivity, electrical conductivity,    mechanical strength, and stiffness (elastic modulus).

The aforementioned features are further described and explained indetail as follows: As illustrated in FIG. 1(B), a graphite particle(e.g. 100) is typically composed of multiple graphite crystallites orgrains. A graphite crystallite is made up of layer planes of hexagonalnetworks of carbon atoms. These layer planes of hexagonally arrangedcarbon atoms are substantially flat and are oriented or ordered so as tobe substantially parallel and equidistant to one another in a particularcrystallite. These layers of hexagonal-structured carbon atoms, commonlyreferred to as graphene layers or basal planes, are weakly bondedtogether in their thickness direction (crystallographic c-axisdirection) by weak van der Waals forces and groups of these graphenelayers are arranged in crystallites. The graphite crystallite structureis usually characterized in terms of two axes or directions: the c-axisdirection and the a-axis (or b-axis) direction. The c-axis is thedirection perpendicular to the basal planes. The a- or b-axes are thedirections parallel to the basal planes (perpendicular to the c-axisdirection).

A highly ordered graphite particle can consist of crystallites of aconsiderable size, having a length of L_(a) along the crystallographica-axis direction, a width of L_(b) along the crystallographic b-axisdirection, and a thickness L_(c) along the crystallographic c-axisdirection. The constituent graphene planes of a crystallite are highlyaligned or oriented with respect to each other and, hence, theseanisotropic structures give rise to many properties that are highlydirectional. For instance, the thermal and electrical conductivity of acrystallite are of great magnitude along the plane directions (a- orb-axis directions), but relatively low in the perpendicular direction(c-axis). As illustrated in the upper-left portion of FIG. 1(B),different crystallites in a graphite particle are typically oriented indifferent directions and, hence, a particular property of amulti-crystallite graphite particle is the directional average value ofall the constituent crystallites.

Due to the weak van der Waals forces holding the parallel graphenelayers, natural graphite can be treated so that the spacing between thegraphene layers can be appreciably opened up so as to provide a markedexpansion in the c-axis direction, and thus form an expanded graphitestructure in which the laminar character of the carbon layers issubstantially retained. The process for manufacturing flexible graphiteis well-known in the art. In general, flakes of natural graphite (e.g.100 in FIG. 1(B)) are intercalated in an acid solution to producegraphite intercalation compounds (GICs, 102). The GICs are washed,dried, and then exfoliated by exposure to a high temperature for a shortperiod of time. This causes the flakes to expand or exfoliate in thec-axis direction of the graphite up to 80-300 times of their originaldimensions. The exfoliated graphite flakes are vermiform in appearanceand, hence, are commonly referred to as worms 104. These worms ofgraphite flakes which have been greatly expanded can be formed withoutthe use of a binder into cohesive or integrated sheets of expandedgraphite, e.g. webs, papers, strips, tapes, foils, mats or the like(typically referred to as “flexible graphite” 106) having a typicaldensity of about 0.04-2.0 g/cm³ for most applications.

The upper left portion of FIG. 1(A) shows a flow chart that illustratesthe prior art processes used to fabricate flexible graphite foils andthe resin-impregnated flexible graphite composite. The processestypically begin with intercalating graphite particles 20 (e.g., naturalgraphite or synthetic graphite) with an intercalant (typically a strongacid or acid mixture) to obtain a graphite intercalation compound 22(GIC). After rinsing in water to remove excess acid, the GIC becomes“expandable graphite.” The GIC or expandable graphite is then exposed toa high temperature environment (e.g., in a tube furnace preset at atemperature in the range of 800-1,050° C.) for a short duration of time(typically from 15 seconds to 2 minutes). This thermal treatment allowsthe graphite to expand in its c-axis direction by a factor of 30 toseveral hundreds to obtain a worm-like vermicular structure 24 (graphiteworm), which contains exfoliated, but un-separated graphite flakes withlarge pores interposed between these interconnected flakes. An exampleof graphite worms is presented in FIG. 2(A).

In one prior art process, the exfoliated graphite (or mass of graphiteworms) is re-compressed by using a calendaring or roll-pressingtechnique to obtain flexible graphite foils (26 in FIG. 1(A) or 106 inFIG. 1(B)), which are typically 100-300 μm thick. An SEM image of across-section of a flexible graphite foil is presented in FIG. 2(B),which shows many graphite flakes with orientations not parallel to theflexible graphite foil surface and there are many defects andimperfections.

Largely due to these mis-orientations of graphite flakes and thepresence of defects, commercially available flexible graphite foilsnormally have an in-plane electrical conductivity of 1,000-3,000 S/cm,through-plane (thickness-direction or Z-direction) electricalconductivity of 15-30 S/cm, in-plane thermal conductivity of 140-300W/mK, and through-plane thermal conductivity of approximately 10-30W/mK. These defects and mis-orientations are also responsible for thelow mechanical strength (e.g. defects are potential stress concentrationsites where cracks are preferentially initiated). These properties areinadequate for many thermal management applications and the presentinvention is made to address these issues.

In another prior art process, the exfoliated graphite worm 24 may beimpregnated with a resin and then compressed and cured to form aflexible graphite composite 28, which is normally of low strength aswell. In addition, upon resin impregnation, the electrical and thermalconductivity of the graphite worms could be reduced by two orders ofmagnitude.

Alternatively, the exfoliated graphite may be subjected tohigh-intensity mechanical shearing/separation treatments using ahigh-intensity air jet mill, high-intensity ball mill, or ultrasonicdevice to produce separated nano graphene platelets 33 (NGPs) with allthe graphene platelets thinner than 100 nm, mostly thinner than 10 nm,and, in many cases, being single-layer graphene (also illustrated as 112in FIG. 1(B)). An NGP is composed of a graphene sheet or a plurality ofgraphene sheets with each sheet being a two-dimensional, hexagonalstructure of carbon atoms.

Further alternatively, with a low-intensity shearing, graphite wormstend to be separated into the so-called expanded graphite flakes (108 inFIG. 1(B) having a thickness >100 nm. These flakes can be formed intographite paper or mat 106 using a paper- or mat-making process. Thisexpanded graphite paper or mat 106 is just a simple aggregate or stackof discrete flakes having defects, interruptions, and mis-orientationsbetween these discrete flakes.

For the purpose of defining the geometry and orientation of an NGP, theNGP is described as having a length (the largest dimension), a width(the second largest dimension), and a thickness. The thickness is thesmallest dimension, which is no greater than 100 nm, preferably smallerthan 10 nm in the present application. When the platelet isapproximately circular in shape, the length and width are referred to asdiameter. In the presently defined NGPs, both the length and width canbe smaller than 1 μm, but can be larger than 200 μm.

A mass of multiple NGPs (including discrete sheets/platelets ofsingle-layer and/or few-layer graphene or graphene oxide, 33 in FIG.1(A)) may be made into a graphene film/paper (34 in FIG. 1(A) or 114 inFIG. 1(B)) using a film- or paper-making process. FIG. 3(B) shows a SEMimage of a cross-section of a graphene paper/film prepared from discretegraphene sheets using a paper-making process. The image shows thepresence of many discrete graphene sheets being folded or interrupted(not integrated), most of platelet orientations being not parallel tothe film/paper surface, the existence of many defects or imperfections.NGP aggregates, even when being closely packed, exhibit a thermalconductivity higher than 1,000 W/mK only when the film or paper is castand strongly pressed into a sheet having a thickness lower than 10 μM. Aheat spreader in many electronic devices is normally required to bethicker than 10 μm based mainly on handling ease and structuralintegrity considerations.

Another graphene-related product is the graphene oxide gel 21 (FIG.1(A)). This GO gel is obtained by immersing a graphitic material 20 in apowder or fibrous form in a strong oxidizing liquid in a reaction vesselto form a suspension or slurry, which initially is optically opaque anddark. This optical opacity reflects the fact that, at the outset of theoxidizing reaction, the discrete graphite flakes and, at a later stage,the discrete graphene oxide flakes scatter and/or absorb visiblewavelengths, resulting in an opaque and generally dark fluid mass. Ifthe reaction between graphite powder and the oxidizing agent is allowedto proceed at a sufficiently high reaction temperature for a sufficientlength of time and all the resulting GO sheets are fully separated, thisopaque suspension is transformed into a brown-colored and typicallytranslucent or transparent solution, which is now a homogeneous fluidcalled “graphene oxide gel” (21 in FIG. 1(A)) that contains nodiscernible discrete graphite flakes or graphite oxide platelets. Ifdispensed and deposited under a shear stress field, the GO gel undergoesmolecular orientation to form a layer of “oriented GO” 35, which can bedried to obtain a layer of dried GO 37 a. Multiple layers of dried GOmay be compressed to obtain dried GO compact 37 b, which can beheat-treated to become a HOGS 37 c.

Again, typically, this graphene oxide gel is optically transparent ortranslucent and visually homogeneous with no discernible discreteflakes/platelets of graphite, graphene, or graphene oxide dispersedtherein. In the GO gel, the GO molecules are uniformly “dissolved” in anacidic liquid medium. In contrast, suspension of discrete graphenesheets or graphene oxide sheets in a fluid (e.g. water, organic acid orsolvent) look dark, black or heavy brown in color with individualgraphene or graphene oxide sheets discernible or recognizable even withnaked eyes or using a low-magnification light microscope (100×-1,000×).We are quite surprised to observe that suspension of GO sheets, even notin a GO gel state, can be used to produce the HOGS that is thick. Inmany cases, compared to GO gel, the GO suspension can lead to betteroriented GO sheets, resulting in a HOGS that exhibits better electricaland mechanical properties.

Further, even though graphene oxide suspension or GO gel is obtainedfrom a graphitic material (e.g. powder of natural graphite) havingmultiple graphite crystallites exhibiting no preferred crystallineorientation, as determined by an X-ray diffraction or electrondiffraction method, the resulting HOGS exhibits a very high degree ofpreferred crystalline orientation as determined by the same X-raydiffraction or electron diffraction method. This is yet another piece ofevidence to indicate that the constituent graphene planes of hexagonalcarbon atoms that constitute the particles of the original or startinggraphitic material have been chemically modified, converted,re-arranged, re-oriented, linked or cross-linked, merged and integrated,re-graphitized, and even re-crystallized.

Example 1 Preparation of Discrete Nano Graphene Platelets (NGPs) whichare GO Sheets

Chopped graphite fibers with an average diameter of 12 μm and naturalgraphite particles were separately used as a starting material, whichwas immersed in a mixture of concentrated sulfuric acid, nitric acid,and potassium permanganate (as the chemical intercalate and oxidizer) toprepare graphite intercalation compounds (GICs). The starting materialwas first dried in a vacuum oven for 24 h at 80° C. Then, a mixture ofconcentrated sulfuric acid, fuming nitric acid, and potassiumpermanganate (at a weight ratio of 4:1:0.05) was slowly added, underappropriate cooling and stirring, to a three-neck flask containing fibersegments. After 5-16 hours of reaction, the acid-treated graphite fibersor natural graphite particles were filtered and washed thoroughly withdeionized water until the pH level of the solution reached 6. Afterbeing dried at 100° C. overnight, the resulting graphite intercalationcompound (GIC) or graphite oxide fiber was re-dispersed in water and/oralcohol to form a slurry.

In one sample, five grams of the graphite oxide fibers were mixed with2,000 ml alcohol solution consisting of alcohol and distilled water witha ratio of 15:85 to obtain a slurry mass. Then, the mixture slurry wassubjected to ultrasonic irradiation with a power of 200 W for variouslengths of time. After 20 minutes of sonication, GO fibers wereeffectively exfoliated and separated into thin graphene oxide sheetswith oxygen content of approximately 23%-31% by weight. The resultingsuspension was then cast onto a glass surface using a doctor's blade toexert shear stresses, inducing GO sheet orientations. The resulting GOcoating films, after removal of liquid, have a thickness that can bevaried from approximately 10 to 500 μm (preferably <100 μm).

For making a HOGS specimen, a desired number of dried GO films (layers)were then stacked and compressed using a roll-press. The resulting GOcompact was then subjected to heat treatments that typically involve aninitial thermal reduction temperature of 80-350° C. for 1-8 hours,followed by heat-treating at a second temperature of 1,500-2,850° C.

We typically chose to work with thinner coating layers (10-50 μm) since,surprisingly, they were found to have a higher degree of graphene or GOsheet orientation. These led to better HOGS structures as wesurprisingly found after some experiments. The data shown in FIG. 8indicates that a lower thickness value of the individual GO coating orcasting layers led to a higher thermal conductivity of a HOGS having afinal heat treatment temperature of 1,500° C. and a final thickness ofapproximately 0.3 mm for all samples tested. For further comparison, wepoured (cast) a comparable GO solution into a bulk mold cavity of 5 cm×5cm×5 cm to produce a GO casting which was dried and compressed under auniaxial stress to produce a GO compact. This compact was subjected toidentical thermal treatments as in those samples shown in FIG. 8. Theresult was quite shockingly different. The bulk structure is highlyporous with constituent GO sheets being very poorly oriented andincapable of chemical merging and linking with one another (large numberof small grains). The in-plane thermal conductivity was only <100 W/mK.These observations demonstrate the unexpected effectiveness of using thepresently invented layer-by-layer approach of producing a fullyintegrated HOGS structure by preparing, stacking, compressing, andheat-treating thin layers of GO. Such a strategy has never beenpreviously taught or hinted.

Example 2 Preparation of Single-Layer Graphene Sheets from Meso-CarbonMicro-Beads (MCMBs)

Meso-carbon microbeads (MCMBs) were supplied from China Steel ChemicalCo., Kaohsiung, Taiwan. This material has a density of about 2.24 g/cm³with a median particle size of about 16 μm. MCMB (10 grams) wereintercalated with an acid solution (sulfuric acid, nitric acid, andpotassium permanganate at a ratio of 4:1:0.05) for 48-96 hours. Uponcompletion of the reaction, the mixture was poured into deionized waterand filtered. The intercalated MCMBs were repeatedly washed in a 5%solution of HCl to remove most of the sulphate ions. The sample was thenwashed repeatedly with deionized water until the pH of the filtrate wasno less than 4.5. The slurry was then subjected ultrasonication for10-100 minutes to produce GO suspensions. TEM and atomic forcemicroscopic studies indicate that most of the GO sheets weresingle-layer graphene when the oxidation treatment exceeded 72 hours,and 2- or 3-layer graphene when the oxidation time was from 48 to 72hours.

The GO sheets contain oxygen proportion of approximately 35%-47% byweight for oxidation treatment times of 48-96 hours. The suspension wasthen cast onto a glass surface using a doctor's blade to exert shearstresses, inducing GO sheet orientations. The resulting GO films, afterremoval of liquid, have a thickness that can be varied fromapproximately 10 to 500 μm.

For making a HOGS specimen, a desired number of dried GO films were thenstacked and compressed using a roll-press. The resulting GO compact wasthen subjected to heat treatments that typically involve an initialthermal reduction temperature of 80-500° C. for 1-5 hours, followed byheat-treating at a second temperature of 1,500-2,850° C.

Example 3 Preparation of Graphene Oxide (GO) Suspension and GO Gel fromNatural Graphite

Graphite oxide was prepared by oxidation of graphite flakes with anoxidizer liquid consisting of sulfuric acid, sodium nitrate, andpotassium permanganate at a ratio of 4:1:0.05 at 30° C. When naturalgraphite flakes (particle sizes of 14 μm) were immersed and dispersed inthe oxidizer mixture liquid for 48 hours, the suspension or slurryappears and remains optically opaque and dark. After 48 hours, thereacting mass was rinsed with water 3 times to adjust the pH value to atleast 3.0. A final amount of water was then added to prepare a series ofGO-water suspensions. We observed that GO sheets form a liquid crystalphase when GO sheets occupy a weight fraction >3% and typically from 5%to 15%.

For comparison purposes, we also have prepared GO gel samples byextending the oxidation times to approximately 96 hours. With continuedheavy oxidation, the dark-colored, opaque suspension obtained with 48hours of oxidation turns into a brown-yellowish solution that istranslucent upon rinsing with some water.

By dispensing and coating the GO suspension or the GO gel on apolyethylene terephthalate (PET) film in a slurry coater and removingthe liquid medium from the coated film we obtained a thin film of driedgraphene oxide. Each film was slit and trimmed into multiple pieces ofdried GO, which were stacked and compressed to form a GO compact.Several GO compacts were then subjected to different heat treatments,which typically include a thermal reduction treatment at a firsttemperature of 100° C. to 500° C. for 1-10 hours, and at a secondtemperature of 1,500° C.-2,850° C. for 0.5-5 hours. With these heattreatments, also under a compressive stress, the GO compact wastransformed into a HOGS.

The internal structures (crystal structure and orientation) of severaldried GO layers, GO compacts each made of multiple pieces of dried GOlayers, and the HOGS at different stages of heat treatments wereinvestigated. X-ray diffraction curves of a layer of dried GO prior to aheat treatment, a GO compact thermally reduced at 150° C. for one hour,and a HOGS are shown in FIGS. 5(A), 5(B), and 5(C), respectively. Thepeak at approximately 20=12° of the dried GO layer (FIG. 5(A))corresponds to an inter-graphene spacing (d₀₀₂) of approximately 0.7 nm.With some heat treatment at 150° C., the dried GO compact exhibits theformation of a hump centered at 22° (FIG. 5(B)), indicating that it hasbegun the process of decreasing the inter-graphene spacing, indicatingthe beginning of chemical linking and ordering processes. With a heattreatment temperature of 2,500° C. for one hour, the d₀₀₂ spacing hasdecreased to approximately 0.336, close to 0.3354 nm of a graphitesingle crystal.

With a heat treatment temperature of 2,750° C. for one hour, the d₀₀₂spacing is decreased to approximately to 0.3354 nm, identical to that ofa graphite single crystal. In addition, a second diffraction peak with ahigh intensity appears at 2θ=55° corresponding to X-ray diffraction from(004) plane (FIG. 5(D)). The (004) peak intensity relative to the (002)intensity on the same diffraction curve, or the I(004)/I(002) ratio, isa good indication of the degree of crystal perfection and preferredorientation of graphene planes. The (004) peak is either non-existing orrelatively weak, with the I(004)/I(002) ratio <0.1, for all graphiticmaterials heat treated at a temperature lower than 2,800° C. TheI(004)/I(002) ratio for the graphitic materials heat treated at3,000-3,250° C. (e,g, highly oriented pyrolytic graphite, HOPG) is inthe range of 0.2-0.5. One example is presented in FIG. 5(E) for apolyimide-derived PG with a HTT of 3,000° C. for two hours, whichexhibits a I(004)/I(002) ratio of about 0.41. In contrast, a HOGSprepared with a final HTT of 2,750° C. for one hour exhibits aI(004)/I(002) ratio of 0.78 and a Mosaic spread value of 0.21,indicating a practically perfect graphene single crystal with anexceptional degree of preferred orientation.

The “mosaic spread” value is obtained from the full width at halfmaximum of the (002) reflection in an X-ray diffraction intensity curve.This index for the degree of ordering characterizes the graphite orgraphene crystal size (or grain size), amounts of grain boundaries andother defects, and the degree of preferred grain orientation. A nearlyperfect single crystal of graphite is characterized by having a mosaicspread value of 0.2-0.4. Most of our HOGS have a mosaic spread value inthis range of 0.2-0.4 when produced using a final heat treatmenttemperature no less than 2,500° C.

It may be noted that the I(004)/I(002) ratio for all tens of flexiblegraphite foil compacts investigated are all <<0.05, practicallynon-existing in most cases. The I(004)/I(002) ratio for all graphenepaper/membrane samples prepared with a vacuum-assisted filtration methodis <0.1 even after a heat treatment at 3,000° C. for 2 hours. Theseobservations have further confirmed the notion that the presentlyinvented HOGS is a new and distinct class of material that isfundamentally different from any pyrolytic graphite (PG), flexiblegraphite (FG), and paper/film/membrane of conventional graphene/GO/RGOsheets/platelets (NGPs).

The inter-graphene spacing values of both the GO suspension- and GOgel-derived HOGS samples obtained by heat treating at varioustemperatures over a wide temperature range are summarized in FIG. 6(A).Corresponding oxygen content values in the GO suspension-derived unitarygraphene layer are shown in FIG. 6(B). In order to show the correlationbetween the inter-graphene spacing and the oxygen content, the data inFIGS. 6(A) and 6(B) are re-plotted in FIG. 6(C). A close scrutiny ofFIG. 6(A)-(C) indicate that there are four HTT ranges (100-500° C.;500-1,500° C.; 1,500-2,000° C., and >2,000° C.) that lead to fourrespective oxygen content ranges and inter-graphene spacing ranges. Thethermal conductivity of the GO gel- and GO suspension-derived HOGSspecimens and the corresponding sample of stacked flexible graphite (FG)foil sheets, also plotted as a function of the same final heat treatmenttemperature range, is summarized in FIG. 6(D). All these stacked andcompressed samples have comparable thickness values.

It is of significance to point out that a heat treatment temperature aslow as 500° C. is sufficient to bring the average inter-graphene spacingin GO to below 0.4 nm, getting closer and closer to that of naturalgraphite or that of a graphite single crystal. The beauty of thisapproach is the notion that this GO suspension strategy has enabled usto re-organize, re-orient, and chemically merge the planar grapheneoxide molecules from originally different graphite particles or graphenesheets into a unified structure with all the graphene planes now beinglarger in lateral dimensions (significantly larger than the length andwidth of the graphene planes in the original graphite particles) andessentially parallel to one another. This has given rise to a thermalconductivity already >440 W/mK (with a HTT of 500° C.) and >860 W/mkwith a HTT of 700° C.), which is more than 2- to 4-fold greater than thevalue (200 W/mK) of the corresponding flexible graphite foil. Theseplanar GO molecules are derived from the graphene planes that constitutethe original structure of starting natural graphite particles (used inthe procedure of graphite oxidation to form the GO sheets). The originalnatural graphite particles, when randomly packed into an aggregate or“graphite compact”, would have their constituent graphene planesrandomly oriented, exhibiting relatively low thermal conductivity andhaving essentially zero strength (no structural integrity). In contrast,the flexural strength of the thick HOGS samples (even without an addedreinforcement) can reach 134 MPa.

With a HTT as low as 800° C., the resulting HOGS exhibits a thermalconductivity of 1,044 W/mK, in contrast to the observed 244 W/mK of theflexible graphite foil with an identical heat treatment temperature. Asa matter of fact, no matter how high the HTT is (e.g. even as high as2,800° C.), the flexible graphite foil only shows a thermal conductivitylower than 600 W/mK. At a HTT of 2,800° C., the presently inventedunitary graphene layer delivers a thermal conductivity of 1,708 W/mK(FIG. 4(A) and FIG. 6(D)).

Scanning electron microscopy (SEM), transmission electron microscopy(TEM) pictures of lattice imaging of the graphene layer, as well asselected-area electron diffraction (SAD), bright field (BF), anddark-field (DF) images were also conducted to characterize the structureof unitary graphene materials. For measurement of cross-sectional viewsof the film, the sample was buried in a polymer matrix, sliced using anultra-microtome, and etched with Ar plasma.

A close scrutiny and comparison of FIGS. 2(A), 3(A), and 3(B) indicatesthat the graphene layers in a graphene single crystal or graphenemonolithic are substantially oriented parallel to one another; but thisis not the case for flexible graphite foils and graphene oxide paper.The inclination angles between two identifiable layers in the unitarygraphene entity are mostly less than 5 degrees. In contrast, there areso many folded graphite flakes, kinks, and mis-orientations in flexiblegraphite that many of the angles between two graphite flakes are greaterthan 10 degrees, some as high as 45 degrees (FIG. 2(B)). Although notnearly as bad, the mis-orientations between graphene platelets in NGPpaper (FIG. 3(B)) are also high and there are many gaps betweenplatelets. The unitary graphene entity is essentially gap-free.

FIG. 4 (A) shows the thermal conductivity values of the GOsuspension-derived HOGS (♥), GO suspension-derived HOGS (▪), stackedsheets of GO platelet paper (♦) prepared by vacuum-assisted filtrationof RGO, and FG foil (x), respectively, all plotted as a function of thefinal HTT for graphitization or re-graphitization. These data haveclearly demonstrated the superiority of the HOGS structures in terms ofthe achievable thermal conductivity at a given heat treatmenttemperature.

-   1) All the prior art work on the preparation of paper or membrane    from pristine graphene or graphene oxide sheets/platelets follows    distinctly different processing paths, leading to a simple aggregate    or stack of discrete graphene/GO/RGO platelets. These simple    aggregates or stacks exhibit many folded graphite flakes, kinks,    gaps, and mis-orientations, resulting in poor thermal conductivity,    low electrical conductivity, and weak mechanical strength.

As shown in FIG. 4(A), even at a heat treatment temperature as high as2,800° C., the stacked sheets of GO platelet paper exhibits a thermalconductivity less than 1,000 W/mK, much lower than the >1,700 W/mK ofthe GO-derived HOGS.

-   2) The GO suspension-derived HOGS appears to be superior to the GO    gel-derived HOGS in thermal conductivity at comparable final heat    treatment temperatures. The heavy oxidation of graphene sheets in GO    gel might have resulted in high defect populations on graphene    surfaces even after thermal reduction and re-graphitization.-   3) For comparison, we have also obtained conventional highly    oriented pyrolytic graphite (HOPG) samples from both the CVD carbon    film route and the polyimide (PI) carbonization route. The CVD    carbon was obtained at 1,100° C. on a Cu substrate. The polyimide    films were carbonized at 500° C. for 1 hour and at 1,000° C. for 3    hours in an inert atmosphere. Both the CVD carbon films and    carbonized PI films were then graphitized at a temperature in the    range of 2,500-3,000° C., under a compressive force, for 1 to 5    hours to form a conventional HPOG structure. The CVD carbon-derived    HOPG was very thin (<less than 1 μm in thickness) due to the    limitation of the CVD process. Other samples were all approximately    300 μm thick.

FIG. 4(B) shows the thermal conductivity values of the GOsuspension-derived HOGS (▪), the CVD carbon-derived HOPG (▴), and thepolyimide-derived HOPG heat-treated for three hours (x) undercompression, all plotted as a function of the final graphitization orre-graphitization temperature. These data show that the conventionalHOPG, produced by either CVD or carbonized polyimide (PI) route,exhibits a consistently lower thermal conductivity as compared to the GOsuspension-derived HOGS (▪), given the same HTT for the same length ofheat treatment time. For instance, the HOPG from PI exhibits a thermalconductivity of 886 W/mK after a graphitization treatment at 2,000° C.for 3 hours. At the same final graphitization temperature, the HOGSexhibits a thermal conductivity value of 1,568 W/mK. That the CVDcarbon-derived HOPG shows a higher thermal conductivity value comparedto the corresponding PI-derived HOPG might be due to the shear lowthickness of CVD film that was easier to achieve higher orientation ascompared to PI.

-   4) These observations have demonstrated a clear and significant    advantage of using the GO gel approach to producing unitary graphene    materials versus the conventional PG approach to producing oriented    graphite crystals. As a matter of fact, no matter how long the    graphitization time is for the HOPG, the thermal conductivity is    always lower than that of a GO gel-derived HOGS. In other words, the    HOGS material is fundamentally different and patently distinct from    the flexible graphite (FG) foil, graphene/GO/RGO paper/membrane, and    pyrolytic graphite (PG) in terms of chemical composition, crystal    and defect structure, crystal orientation, morphology, process of    production, and properties.-   5) The above conclusion is further supported by the data in FIG.    4(C) showing the electric conductivity values of the GO    suspension-derived HOGS (♦) and GO gel-derived HOGS (▪) are far    superior to those of the GO paper compact from RGO platelets (x) and    compact of FG foil sheets (▴) over the entire range of final HTTs    investigated.

Example 4 Preparation of Pristine Graphene Sheets/Platelets (0% Oxygen)and the Effect of Pristine Graphene Sheets

Recognizing the possibility of the high defect population in GO sheetsacting to reduce the conductivity of individual graphene plane, wedecided to study if the use of pristine graphene sheets (non-oxidizedand oxygen-free) can lead to a HOGS having a higher thermalconductivity. Pristine graphene sheets were produced by using the directultrasonication or liquid-phase production process.

In a typical procedure, five grams of graphite flakes, ground toapproximately 20 μm or less in sizes, were dispersed in 1,000 mL ofdeionized water (containing 0.1% by weight of a dispersing agent, Zonyl®FSO from DuPont) to obtain a suspension. An ultrasonic energy level of85 W (Branson 5450 Ultrasonicator) was used for exfoliation, separation,and size reduction of graphene sheets for a period of 15 minutes to 2hours. The resulting graphene sheets are pristine graphene that havenever been oxidized and are oxygen-free and relatively defect-free.

Various amounts of pristine graphene sheets were added to GO suspensionsto obtain mixture suspensions wherein GO and pristine graphene sheetsare dispersed in a liquid medium. The same procedure was then followedto produce HOGS samples of various pristine graphene proportions. Thethermal conductivity data of these samples are summarized in FIG. 9,which indicate that the thermal conductivity of the HOGS produced frompure pristine graphene sheets (presumably themselves being highlyconducting) is surprisingly lower than that of the HOGS from GO sheets(of low conductivity due to high defect population on graphene planes).SEM examination of the samples indicate that the pristine graphenesheet-derived HOGS has poor graphene sheet orientation and has manygraphene sheet kinks and foldings.

Further surprisingly, there are synergistic effects that can be observedwhen both the pristine graphene sheets and GO sheets co-exist in properproportions. It seems that GO can help pristine graphene sheets getdispersed well in a suspension and get them better oriented when beingcoated or cast into thin films. Yet, the high conductivity of pristinegraphene sheets, when properly oriented, helps the resulting HOGSachieve a higher over-all conductivity.

Examples 5 Flexural Strength of Various Graphene Oxide-Derived HOGS

A series of GO dispersion-derived HOGS, GO gel-derived HOGS,stacked/compressed sheets of GO platelet paper, and stacked/compressedsheets of FG foil were prepared by using a comparable final heattreatment temperature for all materials. A universal testing machine wasused to determine the flexural strength of these materials. The flexuralstrength and flexural modulus of the HOGS samples from GO dispersion,HOGS from GO gel, stacked/compressed pieces of GO platelet paper, andstacked/compressed pieces of flexible graphite foil sheets over a rangeof heat treatment temperatures are shown in FIGS. 7(A) and 7(B),respectively.

These data have demonstrated that the flexural strength of the flexiblegraphite foil-derived HOPG increases slightly with the final heattreatment temperature (from 14 to 29 MPa) and that of the GOpaper-derived HOPG (stacked/compressed/heated sheets of GO paper)increases from 23 to 52 MPa when the final heat treatment temperatureincreases from 700 to 2,800° C. In contrast, the flexural strength ofthe GO gel-derived HOGS increases significantly from 30 to >110 MPa overthe same range of heat treatment temperatures. Most dramatically, theflexural strength of the GO suspension-derived HOGS increasessignificantly from 32 to >134 MPa This result is quite striking andfurther reflects the notion that the GO dispersion- and GO gel-derivedGO layers contain highly live and active GO sheets or molecules duringthe heat treatment that are capable of chemical linking and merging,while the graphene platelets in the conventional GO paper and thegraphite flakes in the FG foil are essentially dead platelets. TheGO-derived HOGS is a new class of material by itself.

In conclusion, we have successfully developed an absolutely new, novel,unexpected, and patently distinct class of highly conducting andhigh-strength material: highly oriented graphene structure (HOGS). Thechemical composition (oxygen content), structure (crystal perfection,grain size, defect population, etc), crystal orientation, morphology,process of production, and properties of this new class of materials arefundamentally different and patently distinct from flexible graphitefoil, polymer-derived pyrolytic graphite, CVD-derived HOPG, andcatalytic CVD graphene thin film. The thermal conductivity, electricalconductivity, elastic modulus, and flexural strength exhibited by thepresently invented materials are much higher than what prior artflexible graphite sheets, paper of discrete graphene/GO/RGO platelets,or other graphitic materials could possibly achieve. These HOGSmaterials have the best combination of excellent electricalconductivity, thermal conductivity, mechanical strength, and stiffness(modulus). These HOGS materials can be used in a wide variety of thermalmanagement applications. For instance, a HOGS structure can be part of athermal management device such as a heat dissipation plate for use in amicroelectronic device, LED lighting device, and photovoltaic energydevice.

We claim:
 1. A process for producing a bulk highly oriented graphenestructure with a thickness greater than 0.1 mm, said process comprising:(a) preparing either a graphene oxide dispersion having graphene oxidesheets dispersed in a fluid medium or a graphene oxide gel havinggraphene oxide molecules dissolved in a fluid medium, wherein saidgraphene oxide sheets or graphene oxide molecules contain an oxygencontent higher than 5% by weight; (b) dispensing and depositing saidgraphene oxide dispersion or graphene oxide gel onto a surface of asupporting substrate to form a first layer of graphene oxide, whereinsaid dispensing and depositing procedure includes subjecting saidgraphene oxide dispersion or graphene oxide gel to anorientation-inducing stress; (c) partially or completely removing saidfluid medium from the first layer of graphene oxide to form a firstdried layer of graphene oxide having an inter-plane spacing d₀₀₂ of 0.4nm to 1.2 nm as determined by X-ray diffraction and an oxygen content noless than 5% by weight; (d) preparing at least a second dried layer ofgraphene oxide by repeating steps (b) and (c) at least one time orpreparing multiple sheets of dried graphene oxide by slicing said firstdried layer of graphene oxide; (e) stacking either said first driedlayer of graphene oxide with said at least the second dried layer ofgraphene oxide or said multiple sheets of dried graphene oxide under anoptional first compressive stress to form a mass of multiple layers ofgraphene oxide; and (f) heat treating the mass of multiple layers ofgraphene oxide under an optional second compressive stress to producesaid highly oriented graphene structure at a first heat treatmenttemperature higher than 100° C. to an extent that an inter-plane spacingd₀₀₂ is decreased to a value less than 0.4 nm and the oxygen content isdecreased to less than 5% by weight, wherein said step (f) occursbefore, during, or after said step (e).
 2. A process for producing abulk highly oriented graphene structure with a thickness greater than0.1 mm, said process comprising: (a) preparing either a graphene oxidedispersion having graphene oxide sheets dispersed in a fluid medium or agraphene oxide gel having graphene oxide molecules dissolved in a fluidmedium, wherein said graphene oxide sheets or graphene oxide moleculescontain an oxygen content higher than 5% by weight; (b) dispensing anddepositing said graphene oxide dispersion or graphene oxide gel onto asurface of a supporting substrate to form a first layer of grapheneoxide, wherein said dispensing and depositing procedure includessubjecting said graphene oxide dispersion or graphene oxide gel to anorientation-inducing stress; (c) partially or completely removing saidfluid medium from the first layer of graphene oxide to form a firstdried layer of graphene oxide having an inter-plane spacing d_(o02) of0.4 nm to 1.2 nm as determined by X-ray diffraction and an oxygencontent no less than 5% by weight; (d) preparing at least a second driedlayer of graphene oxide by repeating steps (b) and (c) at least one timeor preparing multiple sheets of dried graphene oxide by slicing saidfirst dried layer of graphene oxide; (e) heat treating said first driedlayer of graphene oxide and said, at least second dried layer ofgraphene oxide or said multiple sheets of dried graphene oxide at afirst heat treatment temperature higher than 100° C. under an optionalfirst compressive stress to an extent that an inter-plane spacing d₀₀₂in said first dried layer, second dried layer, or multiple sheets ofdried graphene oxide is decreased to a value less than 0.4 nm and theoxygen content is decreased to less than 5% by weight; and (f) stackingsaid first with said at least the second dried layer of graphene oxideor stacking said multiple sheets of dried graphene oxide under a secondcompressive stress to form said highly oriented graphene structure,wherein said step (f) occurs before, during, or after said step (e). 3.The process of claim 1, wherein said fluid medium further containspristine graphene sheets and a pristine graphene to graphene oxide ratiois from 1/100 to 100/1.
 4. The process of claim 2, wherein said fluidmedium further contains pristine graphene sheets and a pristine grapheneto graphene oxide ratio is from 1/100 to 100/1.
 5. The process of claim1, wherein said step (f) further includes heat-treating the grapheneoxide mass at a second heat treatment temperature higher than 280° C.for a length of time sufficient for decreasing an inter-plane spacingd₀₀₂ to a value of from 0.3354 nm to 0.36 nm and decreasing the oxygencontent to less than 2% by weight.
 6. The process of claim 2, furthercomprising heat-treating said highly oriented graphene structure at asecond heat treatment temperature higher than 280° C. for a length oftime sufficient for decreasing an inter-plane spacing d₀₀₂ to a value offrom 0.3354 nm to 0.36 nm and decreasing the oxygen content to less than2% by weight.
 7. The process of claim 1, wherein said fluid mediumconsists of water and/or an alcohol.
 8. The process of claim 5, whereinsaid second heat treatment temperature includes at least a temperatureselected from (A) 300-1,500° C., (B) 1,500-2,100° C., and/or (C) higherthan 2,100° C.
 9. The process of claim 6, wherein said second heattreatment temperature includes at least a temperature selected from (A)300-1,500° C., (B) 1,500-2,100° C., and/or (C) higher than 2,100° C. 10.The process of claim 5, wherein said second heat treatment temperatureincludes a temperature in the range of 300-1,500° C. for at least 1 hourand then a temperature in the range of 1,500-3,200° C. for at least 1hour.
 11. The process of claim 6, wherein said second heat treatmenttemperature includes a temperature in the range of 300-1,500° C. for atleast 1 hour and then a temperature in the range of 1,500-3,200° C. forat least 1 hour.
 12. The process of claim 5, further comprising acompression step to reduce a thickness of said highly oriented graphenestructure.
 13. The process of claim 6, further comprising a compressionstep to reduce a thickness of said highly oriented graphene structure.14. The process of claim 1, wherein said first dried layer, said seconddried layer, or said multiple sheets of graphene oxide has a thicknessno greater than 100 μm.
 15. The process of claim 1, wherein said firstdried layer, said second dried layer, or said multiple sheets ofgraphene oxide has a thickness no greater than 50 μm.
 16. The process ofclaim 1, wherein said first dried layer, said second dried layer, orsaid multiple sheets of graphene oxide has a thickness no greater than20 μm.
 17. The process of claim 1, wherein said graphene oxidedispersion or graphene oxide gel has at least 3% by weight of grapheneoxide dispersed in said fluid medium to form a liquid crystal phase. 18.The process of claim 2, wherein said graphene oxide dispersion has atleast 5% by weight of graphene oxide dispersed in said fluid medium toform a liquid crystal phase.
 19. The process of claim 1, wherein saidbulk highly oriented graphene structure has a thickness greater than 0.5mm.
 20. The process of claim 2, wherein said bulk highly orientedgraphene structure has a thickness greater than 5 mm.
 21. The process ofclaim 1, wherein said graphene oxide dispersion or graphene oxide gel isprepared by immersing a graphitic material in a powder or fibrous formin an oxidizing liquid in a reaction vessel at a reaction temperaturefor a length of time sufficient to obtain said graphene oxide dispersionor said graphene oxide gel wherein said graphitic material is selectedfrom natural graphite, artificial graphite, meso-phase carbon,meso-phase pitch, meso-carbon micro-bead, soft carbon, hard carbon,coke, carbon fiber, carbon nano-fiber, carbon nano-tube, or acombination thereof and wherein said graphene oxide has an oxygencontent no less than 5% by weight.
 22. The process of claim 1, whereinsaid steps (b) and (c) include feeding a sheet of a solid substratematerial from a roller to a deposition zone, depositing a layer ofgraphene oxide dispersion or graphene oxide gel onto a surface of saidsheet of solid substrate material to form said first graphene oxidelayer thereon, drying said graphene oxide dispersion or graphene oxidegel to form the first dried graphene oxide layer deposited on saidsubstrate surface, and collecting said first dried graphene oxidelayer-deposited substrate sheet on a collector roller.
 23. The processof claim 5, wherein said first and/or second heat treatment temperaturecontains a temperature in the range of 500° C.-1,500° C. and the highlyoriented graphene structure has an oxygen content less than 1%, aninter-graphene spacing less than 0.345 nm, a thermal conductivity of atleast 1,000 W/mK, and/or an electrical conductivity no less than 3,000S/cm.
 24. The process of claim 6, wherein said first and/or second heattreatment temperature contains a temperature in the range of 1,500°C.-2,100° C. and the highly oriented graphene structure has an oxygencontent less than 0.01%, an inter-graphene spacing less than 0.337 nm, athermal conductivity of at least 1,300 W/mK, and/or an electricalconductivity no less than 5,000 S/cm.
 25. The process of claim 5,wherein said first and/or second heat treatment temperature contains atemperature greater than 2,100° C. and the highly oriented graphenestructure has an oxygen content no greater than 0.001%, aninter-graphene spacing less than 0.336 nm, a mosaic spread value nogreater than 0.7, a thermal conductivity of at least 1,500 W/mK, and/oran electrical conductivity no less than 10,000 S/cm.
 26. The process ofclaim 6, wherein said first and/or second heat treatment temperaturecontains a temperature no less than 2,500° C. and the highly orientedgraphene structure has an inter-graphene spacing less than 0.336 nm, amosaic spread value no greater than 0.4, a thermal conductivity greaterthan 1,600 W/mK, and/or an electrical conductivity greater than 10,000S/cm.
 27. The process of claim 1, wherein the highly oriented graphenestructure exhibits an inter-graphene spacing less than 0.337 nm and amosaic spread value less than 1.0.
 28. The process of claim 2, whereinthe highly oriented graphene structure exhibits a degree ofgraphitization no less than 80% and/or a mosaic spread value less than0.4.
 29. The process of claim 1, wherein the highly oriented graphenestructure exhibits a degree of graphitization no less than 90% and/or amosaic spread value no greater than 0.4.
 30. The process of claim 1,wherein said highly oriented graphene structure contains chemicallybonded graphene planes that are parallel to one another.
 31. The processof claim 1, wherein said graphene oxide dispersion or graphene oxide gelis obtained from a graphitic material having a maximum original graphitegrain size and said highly oriented graphene structure is a singlecrystal or a poly-crystal graphene structure having a grain size largerthan said maximum original grain size.
 32. The process of claim 2,wherein said graphene oxide dispersion or graphene oxide gel is obtainedfrom a graphitic material having multiple graphite crystallitesexhibiting no preferred crystalline orientation as determined by anX-ray diffraction or electron diffraction method and wherein said highlyoriented graphene structure is a single crystal or a poly-crystalgraphene structure having a preferred crystalline orientation asdetermined by said X-ray diffraction or electron diffraction method. 33.The process of claim 1, wherein said highly oriented graphene structurecontains a combination of sp² and sp^(a) electronic configurations. 34.The process of claim 1, wherein said step of heat-treating induceschemical linking, merging, or chemical bonding of graphene oxide sheets,and/or re-graphitization or re-organization of a graphitic structure.35. The process of claim 2, wherein said highly oriented graphenestructure has an electrical conductivity greater than 5,000 S/cm, athermal conductivity greater than 800 W/mK, a physical density greaterthan 1.9 g/cm3, a flexural strength greater than 100 MPa, and/or aflexural modulus greater than 60 GPa.
 36. The process of claim 2,wherein said highly oriented graphene structure has an electricalconductivity greater than 8,000 S/cm, a thermal conductivity greaterthan 1,200 W/mK, a physical density greater than 2.0 g/cm3, a flexuralstrength greater than 140 MPa, and/or a flexural modulus greater than 80GPa.
 37. The process of claim 2, wherein said unitary graphene structurehas an electrical conductivity greater than 12,000 S/cm, a thermalconductivity greater than 1,500 W/mK, a physical density greater than2.1 g/cm³, a flexural strength greater than 160 MPa, and/or a flexuralmodulus greater than 120 GPa.